IMO, don't build that system, just wait for the xbox 360 to come out and buy that. It will be $400 for the premium version and will have better graphics hardware than the current top of the line gaming PCs. Yes, thats right, it will run games faster than athlon 939 paired with radeon x850s or nvidia 7800's, we're talking systems int the $1500+ price range (without monitor). On the other hand, if you're really dead set on the PC as a gaming platform, wait till after both the xbox360 and the ps3 (released march 06) come out. Optimally, you want to buy once computers surpass the upcoming generation of consoles in terms of gaming power. This will eventually happen, but it may be a good 6 months after the release of the ps3.
Remember when the sega saturn and playstation were released in 1995? Those systems ran games better than all the high end pentiums and apple powerPCs of 1995. The PC didn't clearly pull ahead until 1996 with the release of the 3dfx voodoo PCI graphics accelerator. If you wait a year, you'll have a bigger budget and you should be able to snag a good gaming PC that will beat the consoles.
Historically the months prior to a console release have shown to be the worst possible times to build a gaming PC. I would seriously either buy xbox360 or wait it out.