Hello,
i would prefer a Graphic card out of the HD58XX Series for mining. The architecture of the 5000-series is more opitimized for the sort of parallel computing which is needed for mining. And it's also cheaper.
The best MHash/sec per Cost value has, in my honest opinion the 5830, followed by the 5850 and the 5870. I was lucky getting a nearly new 5870 für about 120EUR today at my local dealer. So if you want to have a dedicated mining System: Choose the 5830 perhaps two oder three of them. If you want to game also, then youn would be better off with the 5870 (for the purpose of mining - due to the architectural properties of the 69XX series, these are better optimized for gaming benchmarks, than for parallel computing.
Points taken. The 5850/5870 were on my list, but keep in mind
I want to buy or have a single GPU in the system, otherwise I would go into power issues/costs, cooling issues/costs and performance fragmentation. So I'm trying to get the single best board available, might upgrade again later this year if this goes well, if not, I have some gaming catch-up to do...
I had the 6870 and the 6970 as very optimal cards for Mhash/s and Mhash/J, and these are deals from friends (include 1+ years of warranty on them). I will look into the higher 5870's at around 230 EUR too...