http://www.dangerden.com/mall/metal_stock.asp
The aluminium and the copper together, contacted by water will cause galvanic corrosion. You can run a search in Yahoo for the phenomenon.
As for the rig, tell us:
1-tubing size
2-rad (type, barb size)
3-waterblock design (size, channel size, layout of channel, barb size, construction)
4-Pump (model, in-line or submerged)
5-Pelt?
That's a good start... What are you trying to achieve? "Silent running" or "Lightning fast"?