High end mainstream: Alienware is total crap (way too much flex in case design) Dell's XPS line is decent though not the best.
High end but not very well known: Killer Notebooks will give you everything you want quality and performance (though it will cost you)
Low end: either Dell or HP would be my choices.
Mid range: Asus with their G1
HP is probably the all round best though their video cards max out at the Go 7600. Dell offers a good combination for price to perfomance with the Inspiron line (try to avoid the XPS, they underclock the 7950 to avoid heat issues). Gateway I have little experience with though I've heard both good and bad. Asus has decent quality as does Acer. Try to avoid companies like IBuyPower as I have heard horror stories. Give a price range for the best advice.
I have a HP DV1000 series laptop and it has been almost 2 years so far. I initially payed $1700 for it with the following specs 1GB, 80GB HD 5400 rpm and a Pentium M 1.73GHZ. The only problem that i had with it so far was that the hard drive crapped out and i send it back to HP and they replaced it at no cost (this was after 11 months of owning it).
I would recommend going with a HP since dells are crap and i used to work as a technician at circuit city and Dells we had the most problems with. But again all manufactures use the same parts so there isnt really a big difference.
If i would buy a notebook right now it would be a c2d from HP with vista.