I would get rid of your 8800GTX even though it was once a great card.
Get something else.
Even if you got a 2nd 8800GTX, you miss out in newer OpenGL version, CUDA and Physx also. If you stick to Nvidia cards that is.
Ok, but it certainly seems that I can't both give it up and get a performance increase within my idea of what I can afford to pay.
And why would I replace it if I'm not getting a performance increase?