If you just want to cool the gpu with a block and place ramsinks on the ram then there's not much to choose between the DD Maze 4, Aquaextreme MP-1 and the Swiftech MCW 60. There is a review knocking around somewhere that puts them all at roughly the same performance. I have two of the MP-1's as they are only £25.99 each and a tenner cheaper than the other two.
If you want a full coverage block then you have the DD Tyee GPU Block (£82.16), DangerDen X18 GPU Block (£86.24), Alphacool NexXxoS NVXP-3 (£44.99), EK-FCX1800 X1800/X1900 (49.99).
There are a few other full coverage blocks but the above ones are the best. That EK-FCX1800 X1800/X1900 (49.99) is very good value seeing as it is camparable with the DD blocks.