Well water is fair more effient at cooling, it keeps your load tempatures alot lower then air could even try to do. My video card sits at 500/1300 all day idle at 30c and once i play a game for two hours its at 36c load. Whereas on air it was at 40c idle and 60c load. big differnce, its great for overclocking , on a cold day i can get my video card to 540/1400 stable. So i am guessing you could break 600/1200 with watercooling mate, if you have anything over dual radiators.
Go for watercooling, don't worry about the stock cooler thing , i have watercooled a GT before, broke it and sent it back with the stock cooler on. Ah no worrys.