I have been running two for a few months and have been getting a consistent 1.5 ghash with clocks of 850/300. My rig is running with a flash drive and linuxcoin, all power saving features turned on in bios, non relevant power features turned off. I pull 600 watts average at the wall through my meter. I pay only $.05 per Kwh
Mining Factor 100 at the end of the time frame: 0.19 USD/24h@100MHash/s
Average Mining Factor 100: 0.20 USD/24h@100MHash/s
Power cost per time frame: 21.92 USD
Revenue per time frame: 90.85 USD
... less power costs: 68.94 USD
Hardware break even: 1 day
Net profit first time frame: 68.94 USD
So the above calculation is at today's price for one month on two cards. Now I will be adding two 5970's to this rig and have 4 for another one since I just bought 4 more of these cards from newegg.
They come with a 5 year warranty, so your busted card logic is dead. If they die after 5 years I am screwed. Until then I get new cards whenever one fails at $299 each.
with 4 cards I will pull maybe 1150 watts at the wall, but let's say 1200. That will be roughly $135 per month. add second rig $270 per month...PROFIT after electricity costs.
Will I pay off my rigs in 6 months? No, but I can sell my hardware for EASILY 60% of its value if I choose to. Especially the 5970's since they are still one of the best gaming cards you can get.
One decent price bump to $5 a coin over the next 6 months and I'm in GREAT shape. Not a bad place to be for a hardware/overclocking geek that hates the international banking cartels