I have 2 of the Palit GTX 680 4GB Jetstream cards, which I fitted a pair of Watercool Heatkiller GTX680 blocks to with zero problems. Watercool were the only company, at the time I bought them, who guaranteed that their blocks would fit without problems on the reference 4GB cards.
As I understand it other manufacturers have since done the same with their blocks. I wanted the peace of mind before I bought the blocks for the cards that I wouldn't run into problems when it came to fitting them, hence choosing the blocks from the company that said "Yes, we've test fitted them ourselves to the same type of card." All the other manufacturers were at the time just saying, "let us know how you get on with them."