Are you worried about the windows experience index score or are you having an issue with your graphics card?
I'm also having some issues. Whenever I'm playing AC3, and sleeping dogs at max settings, my fps drops down 25. I don't know why. I tried using my other pc which has athlon ii x2 240, 4gb ddr3 ram, gtx 460, and asus m2n68-am plus, sleeping dogs ran fine.
Here's my specs
CPU: Phenom ii x6 1075t
Memory: 4gb ddr3
GPU: HIS Iceq turbo HD 7850 2gb 256bit
Mobo: Biostar TA 890FXE.
PSU: Corsair VS 550w