Since you already have a 5850, watch prices in coming days, because the 5 series will probably be price lowered, and their production stopped.
They are produced on the same 40nm technology wafers, and the 6 series is going to be cheaper for them to make.
So I would keep your options to AMD choices unless you want things Nvidia offers, physX ,cuda , their 3d. Nvidia drives.
If you sell your current 5850, which alot of people can't or won't do, you could do the gtx 460's or AMD 6850/70
Or the cheapest route, buy a 5850 thats marked down in coming days. Which will give you NICE performance.