Ok, with a slight decrease on GPU voltage and few settings tested, it seems to be stable with ~700kh/s. But, the problem is that mining seems to use all the power from the Windows, and even the cursor movement is quite jumpy. What settings should I try to get the mining going on the background, so that the PC can still used for other applications also?

Would it also be a good idea to use motherboard's internal graphich card for the monitor, instead of the R9?