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.