Is this for gaming? Exactly how much cash do you have? What's the resolution of the monitor?
I'd say that really the only worth while upgrade would be to get an HD 5770 ($150) or HD 5850 ($300). I'd personally spring for the 5850 because it would be more likely to last through a rebuild in the next couple of years.
It is most definitely for gaming! I dont really want to spend more than $280-300 or so (I live in South Africa so our prices are almost double than US'). Monitor max res is 1920 x 1080. I worried that if I spend all my cash on a good card that my CPU will bottleneck it soon...
You're CPU should be fine for a while. It's very hard to get a CPU to bottleneck the GPU. And if that does happen, you can simply buy an aftermarket cooler and overclock the CPU to alleviate the problem.
Of course, the 5850 (which would allow you to play most games at max details at 1920x1080) is expensive, but the extra cost could be justified as you'd be able to move it in to a newer build once you decide it's time to get a new CPU, board and RAM.