There will be a huge improvement, but IMO I wouldnt upgrade to that high of a GPU. The main reason being that you only have a single core A64 3200+ and only 1GB of RAM.
I would upgrade to at least a high end X2, 2GB of RAM, and an 8800GTS.
Either one will be a huge improvement. I would recommend you sticking with the 8800GTS, as it will play any game at very high quality. Anything more is a waste of money (until you upgrade the rest of your computer), and anything less you will have to upgrade when DX10 becomes required. One question: What power supply are you going to use?