Re: Any Riva TNT2 M64 upgrades?
Be careful as some AGP4X motherboards don't handle 8x cards or, at least, those cards which cannot accept 1.5V signals.
The last native AGP card from NVidia was a GeForce 6200 variant called NV44A which is still a decent card for 2002-2003 games. If you don't care for resolution you can even play newer games like Half Life 2 or Doom II based games. I've installed it in an old Pentium III motherboard and it worked fine. A shame budget cards usually come with low quality RAM with subpar speed and performance.
In the second hand market you can find a good variety of better performing cards. Your power supply seems to me a potential cause of trouble as it cannot provide enough current for a decent high powered card.
All considered, a 6200 or maybe a basic 6600 card seems the best (and safest) choice
Pentium Dual E2180 (2GHz, 200MHz FSB)
Abit L95C (The one and only mATX with dual PCIe slots!) + 2x1 Geil Black Dragon PC6400
XFX 9600GSO 384MB
Little freak wonder