You are starting a new business? So I presume you're business will have to pay for the electricity. Doesn't that mean indirectly that YOU are paying your own electricity?

I suggest you would have a look at the ASICS. The hashrate of the cards you specify, 7970, do about 700Mhash. That is (with 3 cards) 2,1 Ghash/sec. That would do almost nothing compared to the ASIC miners which do 50-60Ghash.

And with the expected bitcoin hashrate to go up from 25THash right now to 150THas one month after the ASICS have come to the market, you're 7970 will do nothing anymore... not even a BTC a month!

If you have those cards right now, go for it. If you want to buy them, I wouldn't bet on it. Wrong investment with no ROI at all...