@cons, to avoid mudding up this thread either have your own thread about the issue or pm me about it.
@lag, no worries, i figured you meant that but thought it was a bit open to misinterpretation. agreed that virtual distorts the audio (which is why i dont like it myself) but some people are obsessed with it. remember that "true" surround headsets also use smaller drivers which often dont sound as nice as higher quality larger ones.
@gex,
if you want the advantage in fps games (such as the audiotechnica ad-series has, akg cans or grados) you would lose out on bass. if you wanted bass you have to give up the advantage (basically not having bass = advantage). your choice to make!
going with a closed can for noise isolation also means reduction in soundstage and positional audio generally. while it could help with ambient noise certainly you would be making a tradeoff to get this as well. your choice to make.
not sure if the dt770 is worth it for you or not. its a bit on the bassy side (not a bass cannon) and doesnt give you any sort of advantage for hearing footsteps due to this. it does however have a fairly wide soundstage despite being closed, decent treble response and is comfortable. the 80ohm version is the most bassy, the 250ohm the best sounding, the 32 the most easy to drive. generally unless you plan on using portables it would be ideal to use the 80 or 250 and use a soundcard with them.
it is worth noting that the dt990 is a good price - worthwhile if you could also add an amplifier or soundcard to your setup but that might end up pushing you over budget.
if you wanted closed cans yet a lighter on bass sound perhaps the akg cans are midway between good gaming cans like the ad-series and closed designs. you would get sound muffling and likely the same signature akg is known for which might be what you want.
if you absolutely want surround sound (and i will say that large soundstage headphones sound fine without this generally) you would want virtual surround sound processing as an option. if your motherboard doesnt have it you could use razer surround however this does not sound as good as having either dolby headphone or cmss3d/sbx which you could get from a sound card. the xonar dg is a cheap way to add not only better than onboard audio and a stronger amp but also virtual support if its not included.
we gave you quite a few options so far. i think you need to start making a few choices or decisions.