As a comment after I've written this, this was mostly me spacing out after a stressful day (I tend to write more the less I pay attention, rather than the opposite), so sorry for the verbose post. Hopefully it will help you to make an informed decision about what models and upgrades you would find most worth it (at least, to the best ability I have to inform) without being too daunting?
I'd recommend a Sager build (or any other Clevo-based model, really: many resellers price match), personally: they have a great price to performance ratio, some of the best cooling in the desktop replacement industry, and don't have that overly bulky, trying-too-hard, and quite frankly ugly (in my personal opinion) appearance of certain brands like Alienware and Asus. In addition, they're comparatively simple to upgrade down the line. And, as you've already seen, it's hard to want an Alienware when you can see how much more you can get in a Sager for the same price. Specifically, I'd recommend you look at the Sager 8170 (Clevo 170HM). If it turns out that your friend wants a smaller 15" laptop for its more manageable size, the 8150 is certainly serviceable as well, and offers most of the same upgrades. I'll recommend what upgrades I would include were it to be my laptop for that budget (if only). You'll see conflicts with what Declivever recommended, but I'll explain the benefits as I go.
Firstly, I'm basing this off of the upgrades available for the 8170 on Xoticpc. Different resellers may have different available upgrades. It should be noted that I actually recommend against the desktop CPU SLI GPU model Declivever recommended: in almost every instance, I'd strongly recommend a single GPU against SLI GPUs of the same power (which is to say, given the option between either a 580m, 6990m, or SLI 560m, I'd choose the first two over the last one every time). Support for SLI GPUs can be finicky and lacking, and it's simply easier to get a single GPU. Even when they are entirely possible to make, upgrades to laptop GPUs are so expensive after the fact due to the almost complete lack of an aftermarket that it's not as though you'd feasibly upgrade to a higher-powered SLI GPU later. In addition, these laptop models run quite hot, and have basically functionally zero battery life.
Screen: most certainly upgrade the screen: you've got room in that budget to spare, and the upgraded screens are supposed to be amazing. Matte or Glare are really up to your friend's personal preferences (glare will have clearer, brighter colors whereas matte will, of course, be useful outside or in any high-light glare-inducing environment). You might even consider the 3D screen upgrade just for the 120Hz refresh rate, entirely disregarding the 3D capability (or not: that would be up to your friend), but they come with the major drawback of necessitating NVidia GPUs: which may or may not be a problem depending on how you intend to distribute the budget.
For Processors, in my opinion, the 2760qm qualifies as a worthwhile upgrade, but beyond that the diminishing returns make them not worth it (plus, most of the things your friend will be doing will be GPU limited rather than CPU limited, so even this qualifies as overkill, but it has support for some features that the entry-level quad core does not). At the LEAST, be sure you get the refreshed processor (i.e. the 2670 rather than 2630. Because some resellers may or may not still offer pre-refresh processors, if you go with the 2760, be sure to get the 2760, rather than the 2720)
Thermal Compound: From what I've heard said, upgraded thermal compounds reduce the heat of the system by a few degrees Celsius. While this may not seem like much, additional cooling never hurts, especially with the high-end, heat-producing parts you will no doubt include (it would be difficult to build a Sager for that cost without including high-end components). Help keeping your components cool also means you are less likely to overheat and damage the components, helping the laptop's longevity. Essentially, with that budget and the upgrade being only $40, it's basically a "Why not" (though it may or may not be free with the CPU upgrade, I'm not quite sure: I'd ask an Xotic rep).
Definitely upgrade the GPU. The 6990m and 580m are at roughly the same level performance wise and the two highest performing single-card mobile GPUs on the market at the moment. The 6990 is significantly cheaper, and the 580 offers some extra features. The difference in cost, however (the 6990 is on sale right now for $145 for the upgrade, the 580 upgrade is $475: a total difference in cost of $330, pretty much the cost of a SSD boot drive an a storage HDD), makes you question what value those features have for you.
As for Ram, 8gb should be more than plenty for your friends needs, and even that much should actually be in excess of his needs for most games. Gaming, by and large, is not the most ram-intensive task for a computer (in comparison to, for instance, video encoding). However, I'd keep with the two SODIMM version rather than the cheaper four SODIMM 8gb, simply to make it easier (and cheaper) should he desire an upgrade to 16gb down the road. The 1600MHz is also unnecessary: ram clock speed has a negligible effect on performance in comparison to other upgrades, so it's not really worth the money, to me at least.
The 8170 has two hard drive bays (as opposed to the 8150's single). This allows for a SSD as a boot drive and a larger, slower HDD for a storage drive. The Sager bios apparently has some issues with Sandforce-based drives, hence the lack of widespread brand availability for upgrades (at least, on xotic's site). The 120gb 510 6gb/s drive should prove more than sufficient as a boot drive (and for those more loading-screen-intensive games), while still remaining at a comparatively reasonable price point. Don't go for the 320 series, as they are slower 3gb/s drives. As for the second bay, I remember HDD upgrades in it being cheaper than that, which is odd and could just be a fault with my memory. At the moment, it's about the same price to find an aftermarket 2.5" 7200RPM 750gb hard drive as it is to have xotic install one, so it's up to you which you prefer (the advantage of an aftermarket being that you can choose the specific brand and the drive often gets an independent warranty, the disadvantage being that you have to install it yourself).
While there's a little wiggle room in your budget for Blu Ray should your friend want it (or not, depending on other choices), I don't personally find it necessary or even useful. But then, I have a PS3 to handle Blu Rays...should I ever get one.
The wireless card is definitely something you'll want to upgrade: the stock card is supposed to be problematic (as is any stock card), and any of the upgrades will be more than sufficient. Personally, I'd advise the 6230 for its bluetooth capability (and that few if any situations will truly take advantage of the 6300/1103).
The only other (internal) choice is OS: you can get it without an OS installed if you can find Windows 7 for cheaper than $80. Otherwise, Home Premium should be completely fine for gaming, the only limitation really being the limitation of a maximum of 16gb of supported RAM, but I cannot see that as truly posing a problem.
Any remaining room in the budget you have after making it this far can be sunk into a warranty or pocketed, or you can go back over and see if there are any fluffy upgrades that would be nice, but unnecessary.
Simply for the sake of comparison, the closest comparable Alienware m17x (some upgrades are not even potentially available as Alienware upgrades, such as the high color gamut screens) costs roughly $3000, and you can't configure an Asus to have this level of graphics performance.
Wow...This ended up being really long. @________@ Tldr: get a 8170 and make it awesome.