CPU

Athlon/P4 -Athlon Xp seems to run coooler better L2 Cache

Athlon XP runs cooler than standard Athlon, P4 runs cooler still. "

Video Cards

Ati Radeon 9700 64 mb-128mb I believe is available

stick with 128mb for future proofing

Memory you have ****M-DDR-memory slot-some mobos have both DDR-****M slots-DDR is a bit faster
128-256 memory is nice, at least 128 would be good..

if using Windows XP then get at least 256mb, but don't bother getting more than 512mb unless doing intensive work

Compaq/HP/Dell type sysytems are okay, but the only thing really going for them is "Support". If you don't need the support then a homebuilt type machine is usually better, in terms of upgradeability and parts quality.
I've seen parts from low end manufacturers, relabelled by Compaq and sold in new machines (such as GF4's etc that come with slower ram than an off the shelf retail unit from Asus/Gainward etc) just to shave a few dollars off the price tag.