My friend has a pair of crossfired 5770s which he watercooled. He was forced to go for the XSPC x20 VGA core only blocks (for the reasons pastymuncher has already mentioned).
While they're cheap and do the job I'd advise against it. Those blocks are the worst bit of PC hardware I've ever had the displeasure of working with.
Terrible mounting system, irregularities between "identical" blocks, leaky/wonky threads, lack of gaps between threads for larger sized fittings and leaks between base and top of the block.
The mod for the MCW-60 (from what I read when my friend was thinking of water cooling his cards) involves drilling out the holes in the PCB of the GPU to be slightly larger. Not exactly a risk free option.