I strongly recommend against a second 5850. That card is two generations old, almost three. Problems with this:
VRAM. 5850 only has 1GB of VRAM, with Crossfire you are still limited to 1GB due to VRAM mirroring.
Crossfire issues. Crossfire isn't always problem free - you may notice microstuttering, not all games support Crossfire, and framerate scaling isn't perfect even in games where Crossfire works well; it's only around an 80-90% bump in average framerates but framerates aren't as stable, with less increase in minimums. Single GPU is always preferable to dual GPU unless there is no single GPU to match the performance.
Power consumption. Increased power consumption per performance isn't a good thing even with a strong PSU. Yours isn't, I'd want at least 650W for 5850 Crossfire with CPU overclocking. In your case, CPU overclocking will be required to minimize the bottle neck, and this increases power consumption further.
Heat and noise. Dual card solutions will always be noisier and hotter than comparably powerful single GPU solutions, especially on a motherboard with no spacing between dual slot cards.
Value. Your own 5850 is losing value fast, the sooner you sell it and upgrade to a faster single GPU, the better. At this moment you should be able to get around $75-100 for it.
I'd recommend an upgrade to 7950 3GB with overclocking. Your PSU is easily enough for that and it will actually outperform 5850 Crossfire quite handily. The extra VRAM is a nice bonus.
Is your CPU overclocked already?
What's your gaming resolution?
What's your case?
Do you have a budget?
Are you buying in the US?
Last edited by lehtv; 12-26-2012 at 04:20 AM.