I agree with geofelt, prebuild out of the case and put a Windows build on it. If it goes wrong within the case you know its sothing your have done or have damaged something.
I would assemble the parts outside the case first .
I would test the cards individually first.
When you get sli working and all looks good, then is the time to put the parts in the case.
It is easier to debug or replace parts if they are out of the case for testing.