Don't spend much on it, look through the classifieds for a cheap graphics card.
Micro Mart has a very comprehensive ad section, you'll almost certainly find something there.
My advice to you would be to leave well alone though. You won't get much out of that PC. As you said, it's an old one just taking up space. Do you really think you'll use it?
Take it apart, rebuild it, test it, and learn from it. Use it as a project.. even toy with overclocking.. just don't spend much money on it...please.

You know it makes sense ;)
