Although single gpu cards o/c much better, so I wonder if a 6990 combo could beat four 5870.
Of course it would. Bitcoin mining scales linearly, so a pair of 6990 at 880MHz would achieve 1446 Mhash/s, compared to 1328 Mhash/s for four 5870.
It is kind of pointless to argue which one would be faster when overclocked beyond the factory setting, because the overclocking room varies so much from card to card, and with the type of cooling (air, water, etc).
Are you certain? I'm almost positive two 5870s will beat a single 6990 by a good margin at pure hashing power. Have you seen a 6990 hit over 700Mhash/s?
a question on that subject was brought up, so i answered it.
The point was that even though the 5870 is the worst Mhash/W ratio of the 3 choices mentionned, it's the best Mhash/$, partly cause it can overclock so well when compared to dual gpu cards. This is relevant to the choice of cards imo.
Now, let's admit that four 5870's hash like two 6990, it would cost $700~800 to get the 5870s off of ebay, and $1400 for the cheapest 6990's out there. At worst this is a $600 difference. Assuming the 5870's run at 200W/h and the 6990 at 250W/h (im taking large figures here on purpose) you'd be at a 300W deficit per hour with the 5870's. At an expensive $0.20/kW(exagerating figures again), you'd be paying an extra $1.44 electricity per day compared to the 6990's. At this rate, the 6990's will close the gap in 416 days, or a little short of a year and 2 months. I'd take the 5870's, not to mention that you can build up to it slowly with the profit generated by cumulative card purchase.
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