Except for the RAM, I'm actually fairly impressed with the components Maximum PC picked out: an ATI Radeon HD 4870 at $175, $71 Intel Pentium E5200 2.5GHz Wolfdale, $90 MSI Neo3-F LGA 775 motherboard, $21 2GB DDR2-800 RAM (with a rebate, you can go up to 4GB of RAM for $7 more, which I'd recommend), $25 optical drive and $50 320GB hard drive (for $10 more, you can go up to 500GB). Finally, for the case and power supply, a Rosewill TU-155 II 500 Black, which is a good pick since Rosewill makes decent power supplies (I also like Seasonic, which is what I rock in my PC) and it's only $75 for the whole package. After rebates, that's just $479, although that doesn't include Photoshop or Geek Squad visits.
A good 22-inch monitor can be had for $150 easy, and obviously a keyboard and mouse can be found for under $10 a pop if you really wanna scrimp. For the OS, Windows 7 beta is freeeee. Or you can find a copy of XP lying around somewhere, I'm sure.
Depending on the resolution you wanna roll with, at 1280x1024, Maximum PC got an average framerate of 44.42FPS, and at 1900x1200, a still respectable 36.22FPS.
If you've never ever built a computer before, check out Lifehacker's first-timer's guide to building your own PC. It's a pretty excellent Saturday activity. And obviously a PC can do way more than play Crysis. [Maximum PC]