Tricky. I'm lazy, so I'll just give fairly broad recommendations.
- Get an A64 3500+, 3700+, or 3800+. The 3700+ is preferred for a high-end rig like this, because it's a San Diego and reasonably priced (for a high-end CPU, at least). You don't need an X2 if it's just a games box. It'd be neat to have one, but you wouldn't need it.
- Sound card.. probably an Audigy 2. Also an easy recommendation for a games box.
- Get 2GB of RAM. It's kind of overkill right now, but it won't be that way forever, and your budget has the room. Get something with some overclocking headroom, too.
- 250GB is the current sweet spot for hard drives. Beyond that, there are lots of choices, and it doesn't particularly matter which ones you make.
- You can afford a shiny case, so get a shiny case. The Antec P180 is a really nice case if you want to build a quiet or silent computer, and even if you don't it still has that classy expensive refridgerator look to it. There are many, many, many other choices.
- You probably already know this, but I don't want to take any chances, so: get a high-quality, reasonably quiet PSU. There are many good choices. I'll recommend Seasonic, because 1. Seasonic PSUs are nice, 2. quiet is nice, and 3. I recommended Seasonic earlier and I like recommending the same thing several times in a row. Naturally, there are many other choices out there, and others will be happy to toss their two cents (pence?) in here. Just don't bother with anything rated for more than 450W.
- Get good fans. Panaflo L1s offer a good balance between airflow and noise level.
- DVD burners are cheap enough that there's no reason not to put them even in gaming systems. Also, put in a floppy drive. Obsolete things, but there's no reason not to have one in a standard tower PC, and you never know when you might need one. (Yeah, so this is the bottom of the recommendations barrel. That's why it's near the bottom.)
Last thing: I know you said "no peripherals", but... what monitor are you using? The monitor IS among the most important parts of any gaming rig.