Re: 480 on the way. Power worries with 2nd card as Physx ?
Dedicated PhysX always help even with the 480 GTX. Just look at how the 480 does on the Just Cause benchmarks. Framerates go way down once GPU physics is enabled. One 480 alone can't do 60fps on the Concrete Jungle benchmark at 19200x1200 with max settings.
Just stick with the 8800GT for PhysX though. You'll get 0 benefit from using the 280GTX instead but significantly more heat and power draw. I've benched multiple times with the 280GTX, 8800GTX and 9800 GT as a PhysX card and the results are always the same. My guess is that PhysX just doesn't stress the cards enough because they aren't doing any graphical rendering at all.