Upgrading from there would be effectively replacing almost everything.
As you have already identified you'll need mobo / cpu / graphics card. The new mobo is unlikely to take your existing ram, so you'll need to budget for that too. Since you're buying both mobo and graphics card, you'd be mad not to go for PCI-express versions, and then you'll find you need to buy a new PCI-express compliant power supply too. New mobo will come with sata connectors so you might want a new HDD or two......etc etc
Novatech and the like do upgrade bundles, but with a pc as old as I think yours is, you'd be better off buying a new base unit.
If you want a bit of future proofing I'd look for Intel core 2 duo 6400 or better, a 7600GT graphics card or better and at least 1 GB ram