Best watercooled card for £500

Hi all,

I'm going to be upgrading my old GTX 275 very soon and would like to replace it with a watercooled high-end card. I'm looking for what would be the best value proposition, ie, I don't need to spend all of my money, but if there is a noticeable performance increase it might be worth spending slightly more. I don't care about power efficiency and I'll be overclocking fairly high.

I'm thinking between a 7970, 670 or 680, although the budget needs to take into account the price of the waterblock. I already have a capable watercooling loop installed.

APPROXIMATE PURCHASE DATE: 1 week

USAGE FROM MOST TO LEAST IMPORTANT: Gaming

CURRENT GPU AND POWER SUPPLY: GTX 275, enermax galaxy 1Kw

OTHER RELEVANT SYSTEM SPECS: i7 920 @4.0Ghz

PREFERRED WEBSITE(S) FOR PARTS: Scan, Overclockers, ebuyer

PARTS PREFERENCES: none

OVERCLOCKING: Yes, on water.

MONITOR RESOLUTION: 2560x1440

Thanks!