All i'm trying to say is that the performance increase you'll get after water cooling is not going to be incredibly insane. I'm actually at the same OC with water cooling as I was when I had my 470 with the stock cooler still on. Granted now I could probably push a bit more voltage through and OC a little higher, but the gain is going to be negligible for the money spent.
The only time I would say water cooling would be viable is if you had a motherboard with terrible spacing, a case with bad airflow and you wanted a show piece on your desk. The cards that come with non reference coolers(like the Hawk) already run quite cool and they remain quiet. The guys who really see a difference with water are the ones running 3 580's, because without the water cooling they'd never see temps below 90C. But if you have two non reference 460's running in a case with good airflow your temps are going to remain pretty low and you'll be able to get a respectable overclock while keeping the $200 bucks water blocks would cost in the bank for the next big upgrade.
Really I love my water cooling set up. But like I said, if I had it to do over again I would have chosen the same set up without water cooling, and gotten myself some nice non reference 6950's in crossfire with the money I would have saved.
EDIT: If you really have your heart set on water cooling the cards then I'd suggest going for full blocks instead of the universal ones. Just because it looks nicer and cools the whole card instead of neglecting VRAM which can get quite hot itself.