Free electricity makes everything OK. But is it truly free electricity or is someone else paying it? Those cards might cost around $150-$200 per month in electricity costs to someone. It won't take too long for someone to notice. That will also be loud and put off a ton of heat. I say to go with FPGAs and sell the 7970s if you are not a gamer.
I wouldn't touch a FPGA right now with a ten foot pole, you gotta spend $800-1000 just to get 800mhz.
You can buy single 6990 with almost the same hashing power at half the price of a FPGA right now.(FREE POWER)
In the OP situation depending on how much FREE POWER he can get away with spending. Get rid of your crappy 7xxx series cards, it sounds like you have a Bad MB or BAD PS when that many problems.
For this OP, if he still wants to MINE BTC and since he has FREE POWER(Depending on how much free power he can SPEND) Buy CHEAP POWER HUNGRY 5xxx and 6xxx series cards.
Another point people seem to get forgetting, The DAY BFL ships your card are NOT going to become worthless! There are 2+year old graphics cards that are stil sell for $200+ on ebay.
Its going to take weeks and months to ramp up shipping BFL's new product(if they are even the first to ship ASIC). The difficulty is going to go up pretty fast but its not going to be an instant 2 days and you make 0.001 BTC a day.
The way to calculate profit for this OP is as follows.
A. Purchase Price of graphic Cards
B. BTC Generated Before Blockchain Halving
C. BTC Generated When BFL Ships it first ASIC
D. BTC Generated until its no longer worth the OP Time to Run his computer 24/7
E. Sale Price of Graphics Cards
(A+)-(B+C+D)-(E)
(1450)-(340+100+200)-(1080)=Profit $270
Variables
2.0 ghz at .75 BTC a day @ $10 per BTC
60 days till block half and 90 days till BFL Ships
running the cards for the next 3-5 months after BFL Ships
card holding 75% of there value at time of sale.(I think this card should stay around 300 till 8xxx series is released.
Now run that same thing above but when cheap 5xxx and 6xxx cards
But back to the point return the computer, use your old spare parts computer and put a 6990 in it.
(Buy a 6990 from someone that thinks they are worthless when BFL Ships)
Enjoy your Free Power.
I like working with Odd's , Numbers ,and Financial Math.
I enjoy putting different possible outcomes into google spreadsheet and see how different outcomes can happen and what the odds are behind different theories
People who Buy and Sell on emotion and don't look at hard numbers make people who do so without emotions rich