You won't be earning more that 88$(1) a month, mostly due to stale/invalid shares. Likely you'll make even less than that.
You'll make less than that for sure should you choose a pool with mining fees.
It's crucial that you take the slow-but-steady path to overclocking the card - if you crank it too high and it becomes unstable the amount of downtime might be significant.
I'd go with the 6990 as it's both faster and more energy efficient than a 7970.
Should you choose to proceed with bitcoin mining and purchase a second 6990 card, that PSU of yours will need to be upgraded.
Notes:
(1) This value should be accurate as it's based on my own earnings, scaled proportionally down to fit a 750MHash/s rig.
You'll make less than that for sure should you choose a pool with mining fees.
It's crucial that you take the slow-but-steady path to overclocking the card - if you crank it too high and it becomes unstable the amount of downtime might be significant.
I'd go with the 6990 as it's both faster and more energy efficient than a 7970.
Should you choose to proceed with bitcoin mining and purchase a second 6990 card, that PSU of yours will need to be upgraded.
Notes:
(1) This value should be accurate as it's based on my own earnings, scaled proportionally down to fit a 750MHash/s rig.
Thank you very much. The $88 doesn't seem too bad.
Perhaps I shoud do some more reading on this.