Which do you have - pci- express or agp?
If pci-express - bad idea in my opinion. How much are you getting it for? You could get a 6800GS for 200$ or a 7800GT for 300$ and they both out perform it, I believe
If agp, I still think it's a bad idea. If you want to stick with agp, I'd say just get a 6600GT, it's a very good card for only 150$ or less. Wont be as good as the x850xt (Don't think so atleast lol), but since the x850xt is like 400$, you save 250$ and get a good quality video card still.
If you don't want to get the 6600GT and you DO have agp, post back with your components (CPU, motherboard, etc.)