Okay, since i've decided to upgrade my current system, i discovered that my current motherboard doesn't run AM3 processor, since i was planning to buy a Phenom II X4 955. Also, i'm going to buy a ATI HD5850 to replace old GPU
Yeah I'm sure. My Phenom wasn't even listed on the Gigabyte website supported list. AM3 processors are backwards compatible with AM2/AM2+. You just need to make sure you have the latest BIOS, though I don't think a 140W Phenom may not work because of the power requirements.
I know of one other guy that is running a Phenom II X4 945 on that board too. Maybe purchase your CPU, if it doesn't work then get a motherboard. There's no harm in trying and you might save some money.