in constructed the exact same rig a week ago. let me tell you from experience: the 4 5850s fit nicely onto the mainboard, installed archlinux and am mining since.
with about 1,1 Ghash/s. one problem is the tight packing of the graphics cards, which generate huge amounts of heat. if i could find a way to cool them more, i would probably get 1,25 Ghash, but so far no luck. other than that i got the parts for about 750 euro in germany, but i also didnt need a case and 1 gb of ram are absolutely enough. due to the huge increase in difficulty i fear the rig will lose profitability before it pays for itself. i should have invested the money right away, but now i have a nice mining operation running, thats also a lot of fun