I don't know what cpu you have, but it must be old with that mobo. I think you're probably gonna wanna replace the mobo and cpu. you will have to replace them both at the same time, but you should be able to get a decent cpu and mobo for around $200. I tend to be an intel guy, but you might want to look into amd for that price range. figure out what cpu you want, then find a mobo that's compatible with it. you also need to throw that DDR2 in the bushes, but memory is pretty cheap
then down the road look into upgrading the vid card. at which time you might want to get a new psu as well
then down the road look into upgrading the vid card. at which time you might want to get a new psu as well