To get the ideal RAM for you system is a to select one which matches your bus speed. When you see PC2100 it may not mean much to you, but this type of RAM has a maximum frequency of 266 Mhz. So if your motherboard's bus speed is 266 Mhz, you got the ideal RAM for your system. If you put a faster, like a PC3200 which has a frequency of 400 Mhz, your system won't go any faster::::unless you plan to overclock you bus speed (but that is not always advisable).
Also, you motherboard will only take one or maybe two types of RAM, and you would know this by seeing that the physical slot is different.
Go to this website for more info:
You will have all the information you need.