I don't like the shared 1xPCIE bandwith with the GTX 295 in some boards so it all depend but it is considered as SLI .
I think you'll be a bit disappointed with the ATI vs the 295 so make sure to RMA your 280 is something is starting to cook there but hopefully not and it will be very interesting !!! but 295 vs a 5870... lets see.!!! i think the GTX295 will get over the ATI... but the 5870 is a great single card.
Or maybe i could try Tri SLI with yours

Tri SLI , i can on my Mobo ( 3x16 )

( i do have EVGA 780i FTW on both my systems.. my server running 2x8800 SLI/Q9550) a Q9550 runs better than a Q9650 at the same cruising speed 3.6/3.

Too bad you don't have FSX to benchmark some numbers. but what i can say is that when you guys are at 49 fps in RoF i think i hit triple digits..
The 8800 is still a good card and perfect for physics card.. but limited on the Vram for simming at 1920x1200