Yes, I noticed your cross-post on another forum. Glad you're getting some answers!
I wouldn't worry about the water temps, especially once you've properly spec'd your setup. If you know you've got adequate heat dissipation etc. then there isn't much point IMHO - unless you are a hardcore w/c junkie! I thought about putting some in, but didn't want to restrict the flow of water through the loop (the probes will restrict the flow somewhat I think). Not worth it.
Also, remember that you can monitor your CPU and GPU temps - these are the most important ones to keep tabs on! If your CPU gets too hot, it will reduce its lifetime, and also will start to throttle down (SpeedStep/Overheat protection gubbins) so you'll noticeably lose performance. Run Prime95 on all four cores simultaneously and watch those temps rise! See where the temp stabilises - hopefully around 40 - 60 deg C... too close to 90 deg C and you're in trouble!
The GTX480 GPUs were designed to run hot, so they can handle it (and anyway, there are reports of a watercooled GTX480 running at only ~45-55 deg C, as opposed to ~100 deg C on air!!!). So plenty of headroom there I think.
Please do post a build log, as I'd be interested to see how your build goes, especially as I want to try out w/cing two GTX480s in SLI! (What other forums do you go on?)
Best of luck
4rkon