reason to not sli
lets say you do sli two 8800gtx's and the next generation graphics card comes out 9x series. one card of the 9x series will almost always beat an sli'ed setup of the previous generation.
if i had an sli setup with my two 7950's i wouldn't be better off than one 8800gtx
there is no reason to blow another $5-600 more on an sli setup
-why not pocket that, get one 8800gtx, and have the extra $5-600 to use on the 9x series WHEN it comes out.
and i wouldn't worry too much about near future games if i had an 8800gtx. If can play bioshock all the way up on 1600x1200 max, with 8xS antialiasing, 16x anisotropic, with ONE 7950 then you can surely play crysis and UT3 at high settings with an 8800gtx.
check out tom's vga charts and you will see what i am talking about (about the next generation cards compared to the previous generation in sli)
and i restate- stick with gtx and dont go with ultra. you gain maybe 3 frames more with ultra but blow $100+ more to get it