I'm not easily confused but you have succeeded. Congratulations
I think I'm starting to see how you designed your loop ...
So you have at the moment (please correct me if I'm wrong)
Res - pump - bottom 670 - top 670 - (and this is where it gets confusing)
You have a tube going to the CPU from the top 670 but you use the same port where the you let the water in from the bottom 670. This won't work well at all since I think your CPU block is less restrictive than that GPU (CBA to look it up, but I reckon it is).
What I would do:
Connect the pump to one of the left ports on the bottom 670. Remove the tube going to the CPU from the top 670. The tube on the left side of the top 670 would look nicer if it went to the right port on your rear rad (right as if you were standing behind the case looking at the back). Then you could have a tube going from the left port on the rear rad to the CPU inlet. That's all.
Loop order: Res - pump - bottom 670 - top 670 - rear rad - CPU - top rad - back to res
Connect the pump to one of the right ports on your bottom 670, move the SLI tube to the left. Remove the tube going from the left port on the top 670 to the rear rad. Leave the tube going from the right port on the top 670 to the cpu like it is. Connect the CPU outlet to the right port on your rear rad (right as if you were standing behind the case looking at the back). Connect a tube from the left port on the rear rad to the top rad.
Loop order: Res - pump - bottom 670 - top 670 - CPU - rear rad - top rad - back to res
Personally I think option 1 will look nicer and it's hard to tell from the picture but the connection from the rear rad to the top rad might be a nasty turn in option 2 .
The problem you have like it is now is that you split your loop at the top 670 in 2. This is perfectly possible but not like you did it. What would happen in your design: the water coming from the bottom 670 into the top 670 will hardly go through the 670 block and rather continue to the CPU. If it did flow properly through the block then you would have split your flow in half (or 1/3 - 2/3, no idea really). This could work if you connect the outlet from the rear rad to either the res (unnecessary long tube required) or the top rad using some kind of splitter to have the outlet of the CPU and the outlet of the rear rad go into the top rad's inlet. I'm not a watercooling expert but if this would work then I reckon it wouldn't be optimal.
Absolutely dreadful paint mockup of option 1 (made the overlapping lines a bit more distinguishable.)Edited by Bloitz - 8/14/12 at 12:40pm