There is a good youtube video that shows using an older card as physx and a new one is actually slower than just letting the new one do the graphics and physx. Try removing the older card and try again. Basically the older card is so slow, that it slows down the whole system and becomes your bottleneck.
Seconded on this one. I've seen it too, I think it is by linus. So yes, try removing the older card and hopefully you'll be satisfied.