If your motherboard can accept a Phenom II that would be a more cost
effective upgrade than a new motherboard.
I had a similar ASUS motherboard and CPU which finally seemed usable when
the CPU was upgraded.
Before you buy anything you might want to see if flashing to the newest ASUS
BIOS improves performance as there have been issues. You might want to troll
the ASUS user support groups.
Sadly AMD CPUs simply do not make economic sense for an individual computer
builder as even Phenom IIs do not run Vista as well as comparable Intel Core
Duos and are totally blown away by any Intel Quad core.