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.