I'm not sure a 6600 would require water cooling... are you OCing it? If not, I'm pretty sure even a sophisticated passive cooler (ie Zalamn-style heatpipe) would be enough (assuming your case has good ventillation).
How much is it?
If you really want to get into water cooling then an all-around system (or at least a CPU-VGA set) is a better approach. I'm building my new computer, a new top-notch machine - probably the last one as we most likely will get into the parenting business sometimes next year or so - and since I'm sold my former WC set (excellent Corsair HydroCool200), I've decided to build a super-quiet but high performance water-cooled system and even though I've bought the highest quality parts form everything, it's still not more than a retail WC kit would cost.y