I'd probably shoot for the 6600GT, it's quite a bit faster than the 9800 in almost all cases. If the deal's still going when you buy the card, outpost.com (Fry's Electronics' online store) has a BFG 6800 OC for $199 - a truly awesome deal.
As for the CPU, AMD's trying to kill off the XP line - pricewatch.com only lists 2 vendors with 2500+s in stock and those are mobiles. Mobile XPs are really great chips if you plan on overclocking but if you plan on running at stock speeds, for about the same price, you can get one of the socket 754 Semprons which will run as fast or faster than the 2500+. The motherboards will cost a few bucks more, but it'll probably be worth it.
You might want to check out
http://arstechnica.com/guides/buyer/...e-200504.ars/2 to see what a popular hardware site is recommending for machines at about $800 (including monitor). Trimming some of the edges, it should be really easy to get this to $500 if you don't need a monitor - only major changes I'd probably make are that from the 6600 they use it's really tempting to jump to either a 9800 or 6600GT and that socket 754 Athlon 64s/Semprons are worth considering.