If its a reference design HD6950 then yes (if your not sure just post a link to the exact model your looking at/have and I or someone else will tell you).
Watercooling your GPU will set you back about £230 (£80 for Pump +Res, £80 for GPU block + Fittings, tubing, coolant, Radiator, Fans....) Thats alot of money so I'd only recommend it if you after a quiet GPU and heavy overclocks.
Also you'll have to read up on watercooling alot and be prepared to do some maintaince every once in awhile.