Wow that's an old system. I don't recommend putting in any graphics card to that system. It probably doesn't even have a PCI-express slot.
You need a new system. Not worth upgrading that one, sorry. If you MUST put in a new graphics card, you need to check to see if your motherboard has a PCI-express slot or if it uses the older AGP slot for graphics cards.
If you must upgrade it, I'd say get whatever $50 PCI-e or AGP graphics card you wish. It's still a waste of money in my opinion, but that's the best you can do on such an old rig.
What's your intended use for this system?
If your goal is to play the latest games it simply wont cut it.
But if it's intended to be used only for web browsing, word processing, etc... it should be fine.
If the system doesn't already have a video card, as mentioned before check if you have an AGP slot, if not PCI will be your only choice, it's probably not worth spending too much on so looking for a used card might be a better option.