Ya my 7970 is OC'd to 1200MHz, and I get little graphic glitches and flickers sometimes, esp when using Chrome. It's not bad, and doesn't hurt anything. The only thing you gotta worry about is when your driver crashes or freezes, then you've OC'd too far.

If you can go to 1100MHz in MSI AB, try the same settings in CGMiner. I bet you'll be closer to 550MH/s.

cgminer -o stratum.bitcoin.cz:3333 -u Draxus.worker1 -p C -I 7 --auto-gpu --auto-fan --temp-target 70 --gpu-fan 40-90 --gpu-engine 600-1100 --gpu-memdiff -150

The nice part about CGMiner compared to GUIMiner is it will actively monitor the temperatures to make sure you won't damage your card. The settings I gave you will adjust the fan, and even dynamically lower your overclock to prevent overheating, thus saving your card from damage.