You are only going to be able to run the CPU's that are part of the supported CPU listing. And I'd avoid the ones that are tagged as beta. I'd go with a Phenom II x3 or x4 CPU, but be sure your power supply can handle the extra load. If you are running just a 300watt power supply, I'd install a 500watt power supply to match the extra power demands of the faster and power hungry cpu.
I have a system that i upgraded from a Socket AM2 Sempron x2 2Ghz to a Athlon II x4 620 2.6Ghz quadcore and it was a sweet upgrade. I ended up getting this quadcore for free from a friend who cooked his integrated GPU on a HP tower and when we bought a new motherboard for his system he decided to go with a Phenom II x6 six-core CPU, and gave me his prior quad-core as payment for fixing/building the system up. I was willing to pay $50 for this CPU at the time and he said nah you can have it free. The Athlon II x4 620 performs well and it can be bought used for around $50-60 USD, the Phenom II x3 and x4's are around $75-100 although you have to make sure its supported on the CPU list and I'd avoid those listed with beta indicators which can lead to problems. Phenoms are generally better than the Athlons with the L3 Cache, and even a slower clock Phenom can out perform a higher clocked Athlon, however there are some high clocked Athlons in the 2.6Ghz + clock range that run better than slower clocked 1.8Ghz Phenoms etc. Here is an example:
http://cpuboss.com/cpus/AMD-Phenom-II-P820-vs-AMD-Athlon-II-X4-620 So you cant just assume that anything Phenom is better than Athlon!
Be sure to make sure your heatsink is rated for proper wattage to draw heat away if going with a 95 watt CPU though. I got lucky that my Athlon II x4 620 at 2600Mhz runs 45C to 50C with a 65watt rated cast aluminum heatsink when gaming, but its a 95 watt CPU!!! The prior CPU was a 45watt CPU in which this 65 watt heatsink was plenty, but running a 95 watt ( hotter running ) CPU with a lower rated heatsink can lead to overheating your CPU.
