Dual gpu minimum requirements

Hi, 
I have a 550 W power supply right now and a 9800 gtx+ inside my machine.
My friend has a 8800 ultra lying around and just gave it to me.
I was wondering if I can use the 8800 ultra as the main graphics card and the 9800 gtx+ as a physx card, or vice versa. Would my 550W power supply be able to handle it? and which one would be better to use for the main graphics?
Thanks! 

The 8800 Ultra consumes alot more power than the 9800GTX+. You PSU will stress out if you use 2 cards at the same time. Although I don't think that'll be the case necessarily, so give it a shot. And stick with the 9800GTX+ as a main card.

Thanks man!