This problem maybe your gfx card & drivers supports dx10 xp supports dx9 windows 7 supports dx11...
I have same card(q9550 cpu,win 7), getting 320fps (@2xAA & 16xAS & 1680x1050x75hzx32bit) while stopping bl gp pole position in car view then limiting to 85fps i think thats the ideal for both single&multiplayer setup you can try
I think all games need %5-%10 gpu power surge for multiplayer fps drops...