Depends what are your goals? What do you want to do with this system? Is it for gaming? And if so, what games? What is your resolution?
If you want us to suggest parts, we need to know what you will be using the PC for. Maybe if you only want to play old games at medium settings it could work.
But as suggested by lovesX79 it might make more sense to sell it and use the money to build or buy a new one because it's pretty outdated. As was suggested one of the best CPU you can get would be a Core2 Duo (not sure if it would support a Core2 Quad) but even so, these CPUs are a little outdated already. The other problem is that your PSU is only 250-Watt which limits a lot your options for the video card. So you might need to upgrade the power supply as well. I don't know how much RAM you got in there but most modern games will require 4 GB so you might need more RAM too. So new CPU, RAM, PSU, GPU, might as well get a new PC instead of putting money on an old one. It's pretty much like putting money on an old car, you just want to get it fixed not really upgrade the performance, I wouldn't spend money for a turbo kit and performance tires on a 2001 Hyundai Accent (sorry for the comparison).