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I'm a lil way off from getting my system rebuilt so I can't test em. Just trying to figure out if i'm better off selling em both and using my 8800GT as i've read some inconsistent reviews on their performance.
PhysX scales pretty linearly. A faster card will get you faster PhysX. Check out HD5770 bottlenecked by 9800GT for PhysX. Even with just a 5770, going from a GTX275 to a GTX285 for PhysX improved framerates.
It basically comes down to how much you want to pay (dollars, power, slot space, etc.) and what you consider "good enough". Going from a 9800GT to a GTX275 for PhysX increased average framerates on the 5870 by about 8fps, but only by about 3fps on the 5770. The 9800GT works pretty well with a 5770, but holds the 5870 back more. However, the 5870 was still averaging 98fps even with the 9800GT, which should be more than enough for someone running 60Hz vsync.
The GTS250 will be faster than the 8800GT. Since you already have it, I'd probably just go ahead and use it. If you can get significantly more selling the GTS250, then the 8800GT still isn't too bad.