The forums are dying, so hopefully this goes through.
I run a very similar configuration. I went with Windows 7 as I have more experience setting that up.
I'm running 11.5 Catalyst drivers, 2.4 SDK
Phoenix miner with the following settings
BFI_INT VECTORS WORKSIZE=256 AGGRESION=13 FASTLOOP=false -k phatk
Overclocked via MSI Afterburner after adding the following to the cfg file
UnofficialOverclockingEULA = I confirm that I am aware of unofficial overclocking limitations and fully understand that MSI will not provide me any support on it
EnableUnofficialoverclocking = 1
UnofficialOverclockingMode = 2
AccessibilityCheckingPeriod = 0
Settings in Afterburner are 1025 Core clock, 325 Memory. You can't just bring the memory down to 325 in 1 go, and you can't ever set it between 500-600 or it will lock up. Set it to 650, close afterburner, reopen afterburner, set it to 325.
Fan speeds manually set to 90%
I get 314-317 MHash/s per card. GPU temps are between 53-56C (basement, so ambient is very low)
This is faster than I ever achieved with any other combination of software, miner, and SDK version with these cards. Memory cannot be easily underclocked in linux so you'd have to bios flash the cards using windows first if you went the linux route (if you want the benefits of lower temps and slightly higher MHash/s that comes with lower memory clocks)
If you need any additional help PM me or respond here.
Edit: In windows a dummy plug is not exactly necessary, but it does make things easier. Without a dummy plug you'd overclock 1 card, and start mining on it. Then switch the cable to the other card, overclock it, and start mining on it.
The 30 second dummy plug will work, but the 5 second dummy plug is quicker and just as reliable. http://blog.zorinaq.com/?e=11