Nice looking build - any plans for the same finish on the PSU?
As for the 5850 - the 4870 is exactly the same size - i have a dead one that i used as a template for mine (before i killed the mobo and went back to my HUGE Lian Li)
Something i would sujjest - is you have the H50 the right way, add a strip of tape or something non-conductive to the side of the H50 rad - as with the 5850/4870 in there there is ~1mm to the back side of the card.
Thats a link to my worklog on my sg05.
I ran a i7 860 - but other than that our setups are almost the same (i also managed to fit in 2 3.5" HDDs), and i modding in a full ATX PSU
Hope if gives you some good ideas.
As for the H50, I thought about that issue while I was doing the mockup with the 4870 and applied a little black electrical tape to the side.
Thanks for the advice though man!