I have a very similar system. Athlon 700 on a AMD 751 chipset mobo, 256 pc100 RAM and a Geforce2 GTS.
When I first got win2k in my system (had a voodoo3 then), it started to reboot randomly. Then I upgraded to the GTS, and still had the same problem. When sp1 was realeased, I installed it, and my system messed up so badly that I had to reformat. I did a clean install of win2k, installed the sp1 and new drivers for everything, and now the problem is gone, or at least I haven't had it for about 5 months.
Ah, and also, I don't remember when but I used an "reset configuration data" or something like that in my bios.