not really the gpu is weak for BF3, the CPU is good enough (just in my opinion).
watch out for heat, it works a gpu hard, and you will have heat issues.
It would be better to buy a GPU for that desktop and use the quad. I'm on a Q9550 (a but faster and a GTX470, and I can get decent frame rates at decent quality. 3GB is just about oK. have a look here http://www.tomshardware.co.uk/gaming-graphics-card-review,review-32400-7.html
to see whats a affordable and 4+ brackets above where you are now.
Not really sure why its not working as is though. latest nvidia drivers?