Custard, what voltage are you applying to your E6600 to get you undesirable temps and what temps are you seeing under a full load? Also what are you using to monitor your cpu temps and what case are you using?
Trying to run too many high heat out-put items on a single loop no matter how big the radiator will result in undesirable operation, in my experience a strong flow makes the biggest difference. I am running my e6400, my x1900xtx and my north bridge on a Swiftech H2O-220 Apex Ultra+. It uses high quality water blocks and a very good pump.
However I feel that my water cooling system is near the end of it's effective capabilities. This is why if I were to upgrade to an 8800gtx I feel that the best way would be to remove the vga from the loop keeping the 8800gtx on air. I have a feeling that the added heat out-put from the 8800gtx would overwhelm the system.
Also keep in mind that each item in the loop (cpu block, NB block, vga block and radiator) resualts in a slight but noticeable pressure drop.
Many of the better water cooled systems I have seen cooling either one or two 8800gtx's are running two water loops. (2 pumps, 2 resivours, 2 radiators) With the first loop on the cpu and NB and the second loop dedicated to their graphics cards.
Do your homework before you buy and if it says Swiftech or Danger Den on it chances are it is quality.
Here is the system I started with: http://www.xoxide.com/swiftech-h2o-apex-ultra-plus.html
I upgraded the radiator fans to 50cfm Vantec Stealth fans which are allot quieter and support speed monitoring/controlling as well as installed more flexible tubing from Masterkleer.
My brain has more info on this topic but it would easily double if not triple the size of this post so if you are interested just ask.
Here are some pictures of my water cooling set-up in a case that is too small to install the pump and resivour inside.
Initial set-up:
After tubing reduction (resulted in a higher pressure and better cooling):
A very nice dual loop set-up with 2 8800gtx's: