I think at this point your system is simply showing its age. A full upgrade would be the most appropriate route if you're looking for good performance, but if you're on a budget I'd at least recommend a new video card. I'd imagine anything more than the likes of a 560 would be bottle-necked by your processor, so I'd be careful not to overdo it.
It's not worth upgrading any component in your PC. You're better off buying a new one online (which can be pre-built too for a small fee). Let us know what your budget is and we'll help you from there.
If that's not an option then I recommend the following: