CPUIf you can afford it get a quad core.
MotherboardWithout looking at specs I can't suggest a specific model or brand right now. I love my old ASUS Striker. Get a mobo that is SLI capable, with as many free slots as you can get if you want to use an aftermarket sound card. The on-board chip sets are pretty good but I could not do without my External SB Xfi Pro. SATA support is the norm now and the more ports the better especially external ports for hooking up an nice external terabyte backup drive.
MemoryDitto on at least 4 gig of the fastest speed memory you can get for the mobo you looking at as Vista/Windows 7 will support 4 gig. If your going to run XP (3 gig limit) no biggie as you will future proof your investment. Most gaming based motherboards only have 4 slots so you don't have many choices on memory size and number of modules.
VideoOn video I highly suggest a SLI based setup with 2 of the best cards you can get for the money your trying to spend.
The newest Nvidia based cards running SLI will easily future proof your new rig for future games. My 2 EVGA 8800 GTXs with 768mb - total of 1536mb video ram still play any game at very nice frame rates and they are over a year old.
Hard DrivesThe new Raptors are still your best bet running Raid0 for best performance. (why I suggested a big backup drive). The new raptors are now 3gbs and running raid0 with give you outstanding read/write performance.
Have fun spending money!