Even if the card was recognized properly, you would have a lot of stuttering, as the card would be much faster than the interface, so it would have to slow down the data transfer, which would be seen as stuttering, or sluggish performance
I have the same motherboard, and am running a new Radeon HD 7850 on it, and I'm able to get 50+ FPS in BF3, with all settings on high (with a 3.0GB Phenom X4 940BE). So, the bandwith limit of the PCI-E 1.0 bus doesn't seem to have much of an effect on the real-world performance you'll see.
The game does have a minor stutter while playing (I came across this thread when researching it), but there's no corresponding framerate drop. I'm 99.9% sure that it's related to the fact I only have 4GB system RAM...I expect the issue to clear up when I add a couple more GB of RAM and move to 64bit Windows.