Won't the OP's setup be obsolete when the ACIS boxes zerg the network between now and end of October? Seems like a lot of effort and money to end up with a loss...
...but I'm just a Noob.
Yes, bitcoin GPU mining is long dead. But GPU's are still used (and profitable) for scrypt mining like Litecoin, Feathercoin and other altcoins. Also depends greatly on your local electricity costs.
As a followup to the ops question I was able to get 6 GPU's working on my gigabyte board with a "little" messing around in win7 x64. I'll be getting the exact same board as you next week so I will be shooting to hit 7 too.
http://www.gobitgo.com/articles/1002/How-to-Mine-with-6-GPU/ has similar info that may be a helpful reference. In the link above the author is using windows 8 but the procedure is the same and works just as well in windows 7 x64.
Here's my version of how to get 7 cards working on win7x64:
1) Install 12.6 drivers. - shutdown
2) Unplug all risers, then plug in 1, boot to windows, insure the card shows up in device manager. After its detected and confirmed there, shutdown and plugin the next riser closest (left to right or right to left) and verify in device manager. If the card last plugged in does not show up it most likely means you will need to make a jumper to short the pcie pins. There is a thread on these forums outlining the procedure if needed.
3) Continue verifying all the cards and jumper the ones as needed. After you make it past card 5 you will see cards 6 and above show the yellow exclamation showing a problem. No worries on to the next step.
4) Download 13.1 drivers and double click to extract.
5) Do not install drivers after initial extraction. Click cancel to exit the installer.
6) Back to device manager and we need to update the driver on the devices with the yellow exclamation marks
7) Manually update the driver using the the 13.1 drivers which extracted to C:\amd\13.1... (you can follow the instructions in the link above if you need more specific details)
8 ) This installation/update does take a few minutes so be patient and don't try and close the device manager or updater.
9) On to the next one, same procedure if you have 7 cards.
10) DONE
10) Start Mining already!