if your buying another card, i would just get the same as what you have an add it to your multi-card array.
You'd get more benefit from that, then you would from a dedicated PPU.
Besides, i'm not sure an Nvidia card and an ATI card play nice anymore on the same mobo.
it might be more powerful to sell the 5850s and get something like 2 or 3 GTX 460's/470's because SLI scales so much better than CrossfireX. Of course, only if your mobo can handle it.
2 GTX460s is the limit and will perform worse than the dual 5850s, and consume more power.
3 GTX 470s will cost him ~$500, while requiring a 900W+ PSU, and making his hot like an oven. Besides Crosshair Mobos cant SLI ion the first place, (the hack does work, but thats a different story)
Back to the OP, your 750W seasonic can handle 3 5850s just fine, is it the X-750? Great PSU you got there..
In you still want to go with the dedicated PhysX card, grab a cheap GT240, it should handle the job just fine without having much of an impact on power and heat.