Questions about a pump
Would the Swiftech MPC355 be a powerful enough pump for a 3X120 mm radiator, cpu waterblock (probably oc'ed q6600 for the cpu), and two crossfired HD2X00 series cards. Link: http://www.sidewindercomputers.com/swiftechmcp355.html
At 20.2, it has one of the highest head pressure ratings of most major pumps but, it's discharge rating of around 120gph isn't quite as high as some.
For purposes of definition, “Head” refers to the height of a vertical column of water. This is the maximum height that a pump can sustain any semblance of flow rate before it loses its capabilities. For purposes of an example we'll use a pump rated at 317gph with an imaginery "head" of 36 inches. At 0 inches of height you will have maximum flow rate and the pressure will be zero. Pressure is a measure of resistance to flow. Thus, at its initial discharge, at 0 height, the pump experiences its least resistance and generates its fullest flow. As the height in the cooling loop increases, the resistance to flow increases and the flow rate decreases. Earlier we said that our pump had a "head" of 36 inches. The closer the pump gets to its "36 inch" height, the less flow is generated.
So, at 0 height we have 0 pressure and 317gph. At 36 inches we have full pressure and no flow.
I am certain that the 355 would do fine in that setup but, it might help your overall flowrate if you didn't use fullbody waterblocks for your GPUs.
As far as head pressure goes, you have plenty to spare with the MCP355 and your setup. However, you may find that your flowrates are low. Anything below 1.0 gpm greatly reduces the effectiveness of your cooling loop, and you would be approaching that.
As Phreejak stated, I would also recommend not using a full-cover waterblock for your graphics. That would keep you in the green. If you don't like that idea, then I would recommend using the MCP655/Laing D5 pump.
Would a Swiftech MPC655 be better. Head is lower, but flow is higher. Link: http://www.sidewindercomputers.com/swmc12vdcpuw.html
If it is better, I am willing to spend the extra money on it.