Since you have older Socket A, I wouldn't spend the money upgrading it. I would consider it only if you have Socket 939, which on some motherboards can be upgraded to dual core. But since it's Socket A, the most you could do now is upgrade the memory and video card, which may cost you around $125-150. For around $200 you can get new motherboard, AMD dual core processor, memory and a cheap PCIe video card. You can resuse some of your older components like the case, power supply, etc.
That would be a better use of your money. I just went through this with a couple of computers and I'm glad I did.