If I'm following the specs correctly, your computer has an LGA775 motherboard and an Intel Core 2 Quad processor. If you're going to switch motherboards, you're probably going to have to get a new CPU (and probably PSU) also. Just taking those three components, you're already looking at 50% +/- of the cost of a complete new build. A new build would actually be easier to put together than going through the ass-pain of trying to find compatible upgrades for an...ummm...older OEM system that was originally designed to be an office PC.
We'll help you with appropriate upgrades, but you should seriously think about just replacing the whole thing. It will probably be cheaper and easier in the long run. And the short run, for that...