$850 huh? This is just my opinion on what to get...
p4 2.4ghz cpu
512mb PC2700 ddram
ati Radeon 9500 vid card
Soyo sy-p4x400 dragon Mother board
antec server case
do you need a hard drive? moniter? cd-rom? Or can you just swap those old parts out of your current system? If you can do that then you could get that stuff i listed for about $650 easy!!! If you need a HD you can get a WD 80gb hard drive for about $90 so that would still be $740 and leave you with some extra cash to burn. Maybe on a sweet sound card or something (the mobo comes with onboard sound).