Budget would help, otherwise we cant really suggest anything.
Though I will say, it might be worth just building a new computer outright. By the time your done upgrading that one, a new computer is basically what you will have.
New CPU requires a new Mobo which requires DDR3 RAM. Chances are that HDD is IDE (as well as your optical) so you will need a new one. The new motherboard and HDD requires a new OS install as well.
Depending on the age of the PSU, its probably worth replacing that as well, if it even has the connectors to hook up a graphics card you might get later on.
Basically, to upgrade one thing will have a domino effect and youl end up with a whole new rig anyway.