|Here's the corrected link:|
Your board/chipset is old/outdated & doesn't support modern dual core processors. I don't see much point in upgrading the CPU unless you can find one extremely cheap & even then, I doubt you'd see much of a performance gain.
There were different versions of the GX-280:
The tower accepts up to 4GB RAM ( 4 x 1GB), the SFF only accepts 2GB (2 x 1GB). Regardless of which one you have, you should max out the RAM amount.
Your board should have a PCIe x16 slot for graphics, so if you're still using the onboard video, you may want to add a cheap PCIe video card. If you don't play games or do anything graphics intensive, you won't have to spend a fortune. Intel onboard graphics suck, they steal RAM plus put a drag on the CPU. I wouldn't recommend anything too high end unless you're willing to upgrade the power supply. If you have the SFF system, you'll have to get a low profile card.