I would say upgrade the video card but upgrading the cpu couldn't hurt
PS: So far from experience the 955 seems to handle most games fine
Definitely go with new graphics card.
I've got an amd 955 on an asus m4a88td-v evo mobo with 8 gb of ddr3 and an msi twin frozr gtx 460
Dragon age II on max settings AF and AA all maxed, minus depth of field, and blur effects, hangs around 30-45 fps 1920x1080 res
I would suspect you would be able to achieve roughly the same, with a higher end graphics card.
However, I don't know how much a difference the motherboards will make, and the newer ddr3 ram.
I've also my Phenom II to 3.6 on corsair h60 cooling, as well. Regardless of these inconsistencies, new graphics card,for sure.