The Intel® Pentium® G620 would be the processor to best fit your budget. I would rather see you trying to move to the Intel Core™ i3-2120 but it is double the cost that you state.
Do you have to mobo already? If not tigerdirect has Phenom II x4 955s on sale for $75, I don't usually like to buy OEM chips but thats a pretty good deal. You weren't planning on using the stock heatsink for gaming though were you?