well, you'll need at least a PCI Express slot on your motherboard and depending how old your motherboard is, you might not have one.
Also GTX cards are power hogs, I'd recommend at least a 600w PS.
Finally, GTX cards are monstrous things compared to the old card you have and they produce a LOT of heat, so you might want to check out how big your case is and how good its cooling system is, otherwise you might find yourself without room or worse IMO, with the system overheating.
Now, there is absolutely no point in spending extra for a card like the GTX when you don't have at least a good Core 2 Duo CPU (or equivalent AMX) and at least DDR2 RAM. You should think of upgrading those first IMO. Having said that, your motherboard might be old enough that it doesn't support DDR2 even, so you might find yourself having to scrap everything and start from scratch.
There was a point a couple of years ago where technology took a small leap with PCI express and DDR2 capability, and old motherboards (and even not so old ones) became obsolete quite rapidly. This meant that having to upgrade your computer = buying from the motherboard up. In other words... EVERYTHING! Hopefully, this is not the case with you.
Just my two cents, obviously there's a lot of people in here with more in depth knowledge on the matter. They might be able to explain this better.