when switching vid cards i have always uninstalled the old drivers first.you'll want to first go into vga mode,uninstall old drivers,restart making sure their gone,shut down ,replace card,fire her up and install new drivers.
since your old card was a 128 the 256 series drivers are different.depending on what system you have you should see a desent improvement.exspecialy if you want to run at a little higher res. or at 32 bit color.my old fury would fall on its face in 32bit color.
on a side note ,did you just buy it?if so why not go with the 8500le.price drops mean you can get great deals on them.