I recently bought a second 5850 (reference design) and running happily in crossfire. I'm using MSI Afterburner to overclock the cards (MSI setting to keep same values for similar cards is ticked). Overclocking works fine at 950 core with 1.25v on both cards, but when the cards go back to idle...
I really doubt BC2 will play reasonably, even at low detail settings on that pc at all.
With my Q9400 2.66Ghz quad core chip at 3.1Ghz, I couldn't even fully use my 5850 (oc'd to 5870 performance). I would only get 85% gpu usage as BC2 is so cpu heavy. It required me getting a new motherboard...