Good idea on the cooling situations, but be carefull when cutting these holes. Don't leave your motherboard and CPU in the case (not that you need telling)
When you say 'how bout an 850?', I'm guessing you're talking about watts on a PSU. 850W is WAY too much overkill for your system. I mean you don't even need over 650W to run two 7800GTXs in SLi with a whole range of other power hungry components!! Just stick to 500W maximum, as the 850W will burn a huge hole in your pocket.