I think there are others more qualified than me that can answer that question, but what games are you running? and what resolution/screens do you have?
 

You don't have to, but since you already have the spare, I don't see why not.  The benefit is only there is PhysX enabled games, not all, but you could make your main cards do the job too.  When you say "SLI with a GTX480" do you mean to get two 480's or are you thinking of pairing with the 295?  If so, it won't work.
 
Also, I suspect due to benchmarks like 3Dmark Vantage and 11, people think because physics scores are higher, then frame rates must be higher too with physics enabled on GPU; that's just plain wrongo. 
 
It is true a dedicated processor running physics will be faster than the CPU, but it is like upping any level of deatil, all you're doing is adding more particles for your graphics card to render-- ultimately slowing it.  In other words, enabling PhysX doens't gaurantee an increase in frame rates, even nVidia's website doesn't promise or gaurantee an increase.  The whole point is to increase the level of quality, the eye-candy, to make things look real.  No one says anything officially about frame rates, you get what you get. 
 
My point:  I'm saying don't expect an increase in frame rates with GPU physics enabled, if you get it then hallalujah, but I'm just saying don't expect it as if it was promised to you.  We always get people that claim PhysX is overrated because they expected higher frame rates but they drop.  I've given up wondering why they can't figure it out themselves; its like driving a car, no one has to know how they work to use it; but it would help if they had an idea.

boredgunner

The 8800GTX will provide only little assistance in PhysX intensive games such as Mafia II and Cryostasis, but it may harm your PhysX performance in other applications.  I had a single GTX 480 with a GTX 260 as a PPU at one time, and the 260 harmed performance in a few PhysX benchmarks and added less than 5 FPS max. 

Incorrect, in PhysX games having a dedicated PPU will help performance by a large amount. An 8800GTX will do quite nicely to boost his performance in PhysX games. Take a look at post #16, that test was done with 3 GTX480s and a lowly 8800GS as my PPU and the difference was huge with and without it:
 
http://forums.evga.com...p;m=806660&mpage=1

CraptacularOne

boredgunner

The 8800GTX will provide only little assistance in PhysX intensive games such as Mafia II and Cryostasis, but it may harm your PhysX performance in other applications.  I had a single GTX 480 with a GTX 260 as a PPU at one time, and the 260 harmed performance in a few PhysX benchmarks and added less than 5 FPS max. 

Incorrect, in PhysX games having a dedicated PPU will help performance by a large amount. An 8800GTX will do quite nicely to boost his performance in PhysX games. Take a look at post #16, that test was done with 3 GTX480s and a lowly 8800GS as my PPU and the difference was huge with and without it:

http://forums.evga.com...p;m=806660&mpage=1

 
That's what I would have thought but I was speaking of my experiences.  The PhysX drivers may have been crap at the time but my PPU added about 5 FPS in Cryostasis and lowered the performance in a PhysX screen saver.  It's also widely known that 3DMark Vantage will give you a much lower score with a dedicated PPU.