My system is a 1090T and crossfire 5770's and I have no problem running SC2 on extreme @ 1920x1200. Before I xfired my gfx a single 5770 could run ultra at 30-40 fps.
With the 6850 should be fine running ultra.
PSU: Antec Earthwatts Green 650W Power Supply ATX12V V2.2 EPS12V Active PFC 80PLUS 120MM Fan
Software: Microsoft Windows 7 Home Premium Edition 64BIT DVD OEM
$725.50 CAD including rebates
Any suggestions for improvements would be appreciated
A couple of things... First like some other people have said, your better off waiting for bulldozer for many reasons. I wont relist them. Secondly I don't believe your ram is NOT supported by your mobo, however that doesn't mean it wont work, and in all likelihood it probably still will, however just something to be careful about.
Also I have a i7 860 and a Nvidia GTX 275 with 4 gigs of ram and SC2 runs just fine at full settings so I cant imagine yours wouldn't do the same. However I could be missing something