To be quite honest, you would be better off starting off with new components with the exception of some drives, case, etc. You really need to get a better video card and with that comes a new power supply, and then there is a new motherboard, new CPU, new memory, so by the time you upgrade those, you have a new system.
Take a look at this and evaluate your status, because I think you would be much better off with the cost/efficiency ration to just build new and sell your old on Ebay or something like that:
Want to Build A New Intel or AMD Computer? Here are 6 suggested models with Spec