The fastest CPU your motherboard will support is the
AMD Phenom II X4 975 Black Edition at 3.6GHz
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103923.
but at $150, I think there are other routes to look at not to mention the following disclaimer from Gigabyte... "(Note) If you install an AMD AM3/AM2+ CPU on AM2 motherboard, the system bus speed will downgrade from HT3.0(5200 MT/s) to HT1.0 (2000 MT/s); however, the frequency of AM2+ CPU will not be impacted."
To add any type of decent graphics card, a power supply upgrade would be in order (especially if you go with the Phenom 4 core which can draw up to 125W - without overclock - just by itself)
As far as the graphics card suggestion, start a thread in the graphics card section but explain there what kind of games you want to play and at what settings - give them a budget too, that'll help.