I am currently upgrading from a Nvidia 9800GTX to a Nvidia 580GTX and with the current SLI technology can't have them running together because the GPUs and graphics memory is different on both cards. But a configuration that is supported, is using my old card as a Dedicated PhysX Processor.
I wondered wither this would be better than just letting the 580GTX do all the graphics rendering and physics work. And I found this video when I Googled:
In the video he is using a 8600GTS which isn't as powerful as my 9800GTX, but it is interesting to see that the benchmark scores higher when the 580GTX is allowed to do all the work. I don't think it can be stated strongly enough that the results will change from benchmark to benchmark. But as on overall indicator I don't think I will keep my 9800GTX as even if it could conceivably provides a little extra performance if my 580GTX is under extreme graphics load, the extra heat and power consumption required to keep my 9800GTX makes the whole thing not worth it.