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A 480, it's all you need IMO.
480 is all you need apart from in BC2!
I'd suggest sticking with Xfire 5870s. You'll always make a loss selling them, and despite recent driver advances GTX470s in SLI are only par with them. So, if you do any of the above you'll lose money and get no extra performance!
Keep the 5870s and wait for the 6 series to come out, then sell and get a nice shiny new 6 series card
No real reason for selling
Only the voice in my head keeps chanting nvidia