firstly, are you seriously going to cool a cool card like the 550Ti with water?
second, have you read the sticky in the watercooling section? its in my sig
third, you could go for universal block on your gpu but in all honesty, the GTX550Ti is on par with a HD5770 in both heat signature and performance. overclocking it won't burn your fingers like a GTX580 would.
i mentioned the sticky as there is a small section of how to determine if you want to watercool.