The best processor for your system is an Athlon 64 X2 6000+ Windsor 89w (not the 125w). It's hard to find since they are no longer made, but with the latest BIOS update this processor will work well with your system.
Most of the Brisbanes, which came out after the Windsors, do not work since they are not included in any of the BIOS updates. I have read were some people have gotten them to work by flashing an older version of the BIOS. Personally I would stick with trying to find a 89w Windsor.