A single loop can handle two gtx280's fine provided they aren't paired with a quad core or anything else. After that you either need to think about dual radiators or two separate loops.
LOL! You should see how he attacked my friend who built my first rig! ROFL pure rampage... be careful!
Watercooling is all fine and dandy, and It is not as hard as it seems with the right case. But Veedub is right I think. Build the rig on air, before you do any heavy OCing and such. Research the CRAP out of everything, make a build log and ask questions and people will provid part advice for you when your stuck. After you have everything orderd and on it's way start preparing. Then install!