AM2 socket upgrade

Hi everyone

I'm new here, this is my first post.

I've been trying to get some info about upgrading an am2 socket motherboard: GigaByte GA-M61P-S3
I know that is a old mobo, but I am trying to get some more cpu power for gaming before I get a new pc (probably next year).

I've found that my mobo supports these CPUs: http://www.gigabyte.com/support-downloads/cpu-support-p...

But I don't know which is a better choice, I know that Athlon 64 X2 6400+ (60€ in ebay) is the fastest am2 cpu but I wonder if you recomend me any AM2+ or AM3 cpu.

This is my pc:

Athlon 64 X2 4200+
3GB DDR2 PC2-6400
Nvidia 9800 GTX
GigaByte GA-M61P-S3


I know that I should get a new rig, but I am trying to extend my pc's life a little bit more since COD MW2 and StarCraft 2 are running decently at 1080p (actually starcraft 2 gets slow when there's a lot units on the map).

Im spanish, so Im sorry for my english.

Thank you