Alternatively, if you're comfortable with hardware, I can suggest building your own. You can build a competitive gaming computer for less than half of what you'd pay for the very same shabang from dell or HP.
If you positively need e-peen, you can get something from alienware or falcon or some hyper-expensive custom gaming firm. That's what I did; I bought from Alienware once simply so that I could say "I bought an alienware computer. Rawr." Now I plan to use the wonderful case that I got my alienware in for upgrades. It's humongous, with lots of room for new drives. Plus, it's got some extra fans mounted in strategic locations, for maximum win. However, the gaming OEMs will gouge you even more than the big guys.
Now, onto specifics:
2.3+ Duo/Quad Core Processer
Quad? I can't think of a single game in existance or development (besides Supreme Commander) that even supports a quad core. Though Crysis might, when it comes out. I think you'd be safe with just a dual core. Though if you positively demand top performance, you'll probably have to go with a quad. However, from what I understand, quad cores are incredibly expensive right now.
2GB RAM
Good, but if it's meant to last awhile, you might want to bump that up to four gigs. You can never have too much RAM. It'll let you store all those girnormous fan-made 2800x2800 high-res textures, for maximum prettyness. Plus, more RAM is good for Vista too, because Vista, like any windows OS, hogs RAM nastily.
250 GB Drive
75 GB Backup Drive
Good enough. AZN might suggest a huge HD, but I've found that a mere 160 gig is sufficient for my purposes. And, knowing me, I have a lot of porn. Really, it's hard to watch more than a hundred and fifty gigs of porn, so I think two hundred fifty gigs with 75 gigs for critical files and the OS is perfectly fine.
Havn't gotten a good idea on a card.
8800 GTX. If your motherboard supports SLI, you can eventually get two to run any current and foreseeable next-gen game. I don't even know what ATI's equivelent card is, but from what I've heard, ATI is a ways behind Nvidia at the moment. Though this could change at any time.
Post suggestions, this thing needs to last a while in terms of technology and games.
Don't get cheapo no-name RAM. Shell out for the good stuff. Kingston or Corsair, for instance. Also, get a reliable motherboard. Lots of people go with ASUS, but I've never had any experience with them. Either way, you
really want a good motherboard because if it goes, it can take the whole rest of the computer with it.
Also, you're probably going to need a beefy power supply. 500 watts sounds like a good number, though 650 might be warranted too. That way you'll have enough for any concievable upgrades you might make, too.