Physx is only notable in a very few games (Batman Arkham City being the only one that comes to mind)
I have a 5770 and a empty PCI-e slot but have no desire to run hacked drivers and try to setup Physx with my 8800 GTS 512 (basically same card as a 9800 gts)
The 8800 GTS 512 is only slightly slower (<10% I believe) than the 8800 GTX, because the 8800 GT and GTS 512 used the g92 (which is what the 9800 series and the GTS 250 use) but the 8800 GTX uses the old g80 gpu (same as the 8800 gts 320 and 640 mb)
You can't dual run with an ATi/AMD card and a NVIDIA card. They'd both have to be of the same brand.
Most prefarably from the same series, but you can do crossfire/&SLI from cards from different series - although that could cause complications with drivers and so on. So cards from the same brand and series are the best option to aviod issues.
I was running a GTS 450 and a HD 5570 in the same computer fine