The Athlon x2 is a quite old CPU arch and does not compare favorable to the newer Intel or AMD CPU's.
Your Motherboard is also quite old and does not support any modern CPU's.
If you are on a budget, I would recommend looking into a new AM3 motherboard and an Athlon II or Phenom II CPU.
For around $250 you could get a new platform that is MUCH faster than what you currently have and has room to upgrade in the future.