Here my experience with Windows7 64 bit AMD Driver Support

Setup:
MSI Z77A-G45
6x HIS 7950 IceQ X2
LEPA 1600W NT
4GB G-Skill Ripjaws
40 GB Seagate HDD
old: 4x X1, 1x X16 Riser new: 3x powered x1 Riser + 3 nonpowered x1 Riser + 6x shortener

old: Actual 5 cards are working stable with Win7 64bit, 13.4 AMD driver, AMD APP SDK v2.7
new: Actual 6 cards running after using an x1 riser instead of x16. In addition I undervolted them, to get power consumption down to 95% of power supply range.

At the beginning windows just recognised 4 cards with fresh 13.4 install.
Then I read, only 12.6 would handle with 5 cards.

My Steps:
uninstall 13.4 - reboot - install 12.6 - reboot - 13.4 install (without uninstalling 12.6) reboot

Result: Some configs from 12.6 seems to be persistent, so it still supports 5 grakas.
I dont know if some 13.4 features are disabled by this steps but well.. it works stable.

Here my cgminer setup for 595-600 kh/s:
CoreClock: 1003 Mhz
MemClock: 1490 Mhz
Powertune: 20
ThreadConcurrency=21712 Intensity=19 GpuThreads=1 Vectors=1 Worksize=256

added: gpu-vddc: 1125 (56W less powerconsumption for each device! + ~15% cooler/quieter)