Here's another way to look at the OP's investment. He could have spent $8000 on bitcoins. I'm sure they'll be $13 again this weekend, so I'll use that as the price.
8000/13 = 615
At 7 gigahash the OP should be mining around 3.5 bitcoins a day, at a power cost of at least .5 btc/day. Now assuming no difficulty increases we're talking 200 days to earn that many BTC.
Look at prices and difficulties right around the end of last year (200 days ago) to guestimate how likely it is your hardware will *ever* generate anywhere near 600 bitcoins over its lifetime. You've just bet 8000 dollars no ASIC of FPGA miners get introduced until well into next year, and the 7 series radeons won't be very good at mining -- both very poor bets.
the op made a sloppy investment. You should be able to get at least 1.3 megahash per dollar easily. He gets .875
$8000*1.3=10.4 gigahash
Under current difficulty that's 6.693 BTC I assume that .5 btc is still acceptable because some of the slower more expensive cards consume more power.
So 615/6.193 a day =99 days
So in about 3 months he will have the same amount after that he will continue to gain more bitcoins then the original possible investment.
edit: If for example he went with the most extreme case he could get 1.58 megahash per total cost with 5830's at 110.
$8000*1.58=12.64 gigahash
thats 8.134-.5=7.634
615/7.634= 80 days
