You shouldn't do the mistake only buy one new component, even if you're short on money. Because in a few months or years you have to spend much more money. The reason is that your system is very old, almost all hardware components you own changed it's standard.

If you want to replace one of your components with todays hardware you must either buy slower hardware because your mainboard doesn't support newer ones or buy a complete new mainboard and new components.

An example: If you want a new (better) processor you need a new mainboard. But most payable ones doesn't support DDR and AGP anymore, for e.g... so need also new RAM (DDRII for example) and a new video card (PCI-Express).

This is very variable and flexible at all. In your case the video cards is the worst of your components. You could replace it, but I think it's the best solution to buy a new computer.