Splortch:
I have nearly the same setup. I have a Abit BH6 revision 1.0, CeleronII 566 overclocked to 877, 256 MB RAM, and am running Windows 98 SE. I also got the card from DecoY, and it runs fine with no problems... even ran an entire night of Q3 timedemos to test for stability.
The Geforce2 GTS works fine on the BH6... and yes, BH6's allow voltage adjustments. I really think either your motherboard is bad or you got a bad card.