will psu suffice

[wessa1]

Cooler MASTER Real Power Supply Pro 850W Modular PC PSU
running 2 x 8800gtx sli
4 gb corsair
q6600 @ stock with stock cooler
msi p7n diamond
win xp

dare i add another 8800gtx for tri -sli,or do i really need the massive wattage
that nvidia recommends?already have the extra card from another machine.
wessa1

 

[semi-pro waster]

I'd probably just try it and see, the PSU is from a solid enough brand so if it can't cope with the extra demand then it should just shut down gracefully.