If the purpose is gaming performance then getting a 5850 GPU is the way to go. Your processor may not let you make full use of its power, but it will still be a strong card in a couple years when you do swap out for new CPU/mobo/RAM.
*I better qualify this. If you are running a low resolution monitor, then the 8800 may already be maxxing it out and you would see no difference.
If the purpose is rendering software, video conversion, CAD or other processor intesive apps then the CPU/mobo/RAM will be the thing to swap.