The best for a am2 board was the x2 6400+ that was discontinued years ago. Trying to use a am2+ cpu on it will actually DECREASE it's performance on that motherboard. But the am2+ are also DISCONTINUED years ago. Even if you happen to find a 6000+ cpu, it will set you back 100-200 bucks (NOT even close to worth it). There isn't that much diffference between a 5000+ and a 6000+.
Better saving up your money and getting a new system instead. Look at the cpu benchmarks on this site.