You would see some significant performance increases with the Athlon II X4 640.
Although Anandtech doesn't have the 640 listed in their benchmarks they do have the faster 645 or the slower 635.You can interpolate the results.
First the Athlon II X4 635 vs Phenom X3 8650http://www.anandtech.com/bench/Product/25?vs=122
Next the Athlon II X4 645 vs Phenom X3 8650http://www.anandtech.com/bench/Product/25?vs=188
The Athlon II X4 640 will be between these two.
Now you have a AMD CPU and a 785G motherboard you can go here and download AMD's Overdrive Utility which will let you overclock in a simple wayhttp://game.amd.com/es-es/drivers_overdrive.aspxhttp://www.brighthub.com/computing/hardware/articles/30618.aspx
Otherwise as dogman_1234 suggested you can go to the right section to overclock
in BIOS the manual wayhttp://www.tomshardware.com/forum/forum-29-184.html
or here for various forumshttp://www.tomshardware.com/forum/