CPU: AMD FX-57
Price: $1,011
Single CPU core performance for gaming will still beat the dual cores anyday. And when most games do finally catch up to the hardware, this Socket 939 chip can easily be replaced with an X2 series.

Motherboard: ASUS A8N-SLI Premium ATX AMD Motherboard - Retail
Price: $165
THE motherboard for gamers right now. SLI support and a ton of other goodies. And the PCI-E support on this board gives you the full x16 speed for both slots, as opposed to splitting it up into two 8x slots like most other boards.

Video Card: 7800GTX 512mb (2 of them in SLI)
Price: $1500 total
These cards kick major ass. There really is nothing else to say. SLI wins benchmarks over and over again against the CrossFire series, and nVidia's latest card trumps the X1800XT. If you can afford, you should get it.

Ram: Well, you want to get something with low latency. Probalby two 1 gig sticks to start with. DDR400 should be more than enough, and it is the maximum that board will support. Get four gigs if you really want to.
Price: ~$400 for two 1 gig sticks

Harddrives: Raptors are nice. Don't put them in Raid though, as that will actually decrease their performance. Your best bet would be to get a 74 gigabyte or 36 gigabyte Raptor drive for your OS, and then three or so larger SATA drives like Seagate in Raid-5 for data et al.

Get a good powersupply too. OCZ is pretty good. You'll want to make sure you spend some money on this piece, as it can be the most crucial part of the system. And make sure it is SLI certified; you'll want something above 500 watts.

Save some for a liquid cooling system and a good case. You'll want liquid cooling because that system will be very loud with fans.

Don't forget a monitor. 24 inch LCD from Dell maybe? Get one with a low latency, 12ms or less. And DVI input.

Err, that's all I can think of right now.

And that buddy, is one kick ass gaming system.