Best is to replace both with a matched pair. If you can swing 2GB, it might extend the life by the PC a year or 2 but in the end you will need to upgrade anyway, so consider carefully. Anything less than that is really just a waste of time.
If you do just do one stick, just be aware that the speed of the slowest RAM will be used, so in some cases you would be better off by replacing both sticks with a single stick, rather than leaving one of the older sticks in.
Either way, go to
www.crucial.com and either manually choose your computer model from the list or download the scanner and it will tell you exactly what you can have. At that point, they tend to have better prices on older RAM, but be sure to compare with Memory Express at least.