I have a GeForce 6800 GT and an AMD Athlon XP 2400+ with 768 MB of RAM and Mupen works just fine, but I guess it needs less. I think a GeForce 2 with a 1,7 GHz processor would be enough.
Are you sure you have a GeForce 6800 in your PC? Your processor would be too slow for that card. Or did you mean FX 5800? The 6800 is no FX anymore. Anyway, I guess it's your processor.