A 6790 won't be much more than a 5770. To maximize your speed, tho, make sure you are NOT using an APP SDK 2.6 or newer. APP SDK 2.5 is your best bet.
Try these settings:
--temp-target 70 --auto-gpu --auto-fan --gpu-fan 25-80 --gpu-engine 900 --gpu-memdiff -150 -I 8
Just curious, is a 6790 able to do a --gpu-memdiff of 150 without reverting to default mem clocks? Do not know about the whole 6xxx series but my 6990 is only able to do 125.
I had to flash the bios to go lower.