Personally i went with 3/8" for both GPU & NB. I found that my NB (NForce2) produces as much heat (if not more...) as my GPU (a Geforce2 GTS). When they were air cooled, the GF2 had a smaller heatsink...
Basically it depends on your mobo. Some chipsets seem to produce more heat than others. Nforce2 seems to be quite hot indeed.
DangerDen Z-blocks seem fine (if you're in the US) or if you're in Europe get those Aquacomputer Twinplex blocks. If you really want to, get the GPU block in 1/2" and the NB in 3/8". But staying in 3/8" for the GPU is enough, according to my own experience...
As for RAM waterblocks, it's a scam. My Corsair XMS3200 is *cold* to the touch, not even barely warm. Note that i have *zero* airflow around them. And it's running at full 200MHz with aggressive timings. Save your bucks for something else
