I did something similar.....(not sure of cost so far):
Asus Rampage Extreme III
i7 990x @ 4.3GHz
12Gb (6x2) G.Skill Trident 2000MHz Ram
4 x 60Gb SATAII SSD's (Raid 0) for Windows 7
2 x 2TB SATAII (Raid 0)
2 x 2TB SATAII (Raid 0)
1 x 60Gb SATAII for Windows XP
4 x MSI GTX680's (3 in Tri-SLI & 1 Physx) @ 1200MHz
Five seperate Water Cooled Loops (1 each for the CPU/MB and the GPU's) each with own pump, res, rad and fans (10 fans in total - Scythe Gentle Typhoons 1850rpm)
All powered by a Silverstone Strider 1500w PSU in a Corsair 800D case.
Pictures if you're interested:
http://forums.overclockers.co.uk/showpost.php?p=21799698&postcount=125
wow that must have cost a lot how do the 3+1 GPUs work?