"decent" means lots of things because it depends on the rest of the computer.
Any Core 2 Duo processor will be better than what you have now. What you want depends on what you do with the computer. A Quad-Core is no good if all you do is write letters to the bank.
Cpu Support List for your board is here:
http://www.gigabyte.com.tw/Support/Motherb...?ProductID=3065
A low-end Core 2 Duo could be the E4500, a mid-range/cut-off "value" could be an E5200 or an E7200, and from there you're going off into Quad-Core technology.