very general question. but the best things to upgrade, assuming you have a decent mother board that will allow modern parts, would be components that have processing power, IE, CPU, GPU and RAM.
I would buy a good aftermarket heatsink and overclock the cpu for everything I could get out of it. It's not worth buying a new cpu for your motherboard. And Abit is not longer in the motherboard business so it's unlikely that you'll even have support for the newest cpu's that are available for the LGA 775 socket. But if you get a good overclock you have close to or just as much power as a new cpu anyways.
Does your mobo support Crossfire (does it have 2 pci-e slots)? You could buy a 2nd 4890. Or you could buy a new DX11 video card.
I still think a good heatsink would be the best bang for the buck. And they sell an optional LGA1366 bracket if you do upgrade the whole PC a little later.