pretty much the only thing re-usable on that system if you wish to upgrade is the case and CD/DVD drives, everything else inside it will have to go.
I will assume since you want a PCI-e (or AGP 8X) graphics card, that you are interested in gaming, in which case a celeron processor is useless.
The demension3000 is a budget PC, only capable of basic, non-system intensive tasks, if you need more power, you pretty much have to get a new system altogether.