Good post Grieg.
I really agree to doing it right. Spend the money now and be amazed I say.
Go cheap, be bummed, redo a water loop which is a pain in the hinie and more work than air for sure. And spend more money on bigger rads.
First thing is the case decision. if it never ever moves you can make a lot of stuff external. If you plan to move, do a lan party, will need to move the PC etc just buy a huge case. Here is a neat link someone BLESS THEM made of tons of different builds with many different cases.
Take your time, it's a LOT to digest.
http://www.xtremesystems.org/forums/showthread.php?t=223835
Next, on to the rad and pump situation. Whats your room ambient temps gonna be? I know you know what a hot summer day does to CPU temps on a big overclock. Well watercooling has the same issue. So we compensate for summer by going big on rads to keep the Delta T (Ambient water temp vs air temp) to as low as possible, we shoot in a performance system for 5C or less diff between the two temps. A CPU needs low temps, a GPU loop can go with a 10C Delta T.
So, go as big as you can on the rads. Many who push an i7 realize a quality 120x2 rad is enuff, they wish they had a 120x3 though. For your GPU's I really think a 120x2 would be just enough, GPU's we try to keep cool but get rid of the freakin noise mainly.
TWO LOOPS, no other way with a i7 you plan to OC and TWO GPUs. No other way. You go with one 120x4 rad, your CPU will get warmer water by a few C and warmer water is worse at removing heat than warmer air.
Many decent rads out there, cases, pumps, and blocks. Grieg gave ya a decent list, and since your experianced at overclocking, you know it can't be done or built in a day. I gave general input, but it should help you start.
Plan on pump, 120x3 rad, res for CPU
Plan on Pump, 120x2 min, res for GPUs (I like full cover blocks but a MCW 60 setup is $40+ cheaper than a FC block). FC blocks are the bomb.
Plan your future PC with those basics and then buy the case. If you never ever move the PC, what about a tech station? My rig linky........
http://www.ocforums.com/showthread.php?t=604016