I agree with Radical Dreamer about AMD systems. I have 10 years knowledge of computer systems and build my first Athlon machine in no time at all. This was only after extensive research (which I do with any major purchase) on Athlon systems (Thanx to WinDrivers for most of that).
However, in this case I would recommend a good Pentium 3 upgrade, as Athlon's are murder to troubleshoot (as well as needing specific power supplies).
Ideally to start with try a P3 1Gig, 128 Mb PC133 Ram and a branded Intel 815 mobo (I have had no trouble from Asus and Abit). As long as the power supply is 250 Watt+, there should be not problems.
