Why is the card not at its optimal performance?(Nvidia 7800 GS)Details included?

i have recently bought a AGP 4650 Radeon card to help my gaming needs well i study game play to see what i am to expect i was thinking it was going to be able to game just fine in in's Creed & Oblivion only to find the people with almost the exact same computer specs as me have wayyyyy better frame rates than me is there some thing im doing wrong (i dont have any viruses so dont comment on that.)