Wow, that stuff is pretty old. I don't think $75 is really going to get you very far. You'd want to probably upgrade the motherboard as well if you were going to upgrade the RAM, because PC2100 is pretty slow, and you only have two slots for DDR SDRAM.
If that's your budget though, nothing you can do about it. Go with the RAM istonian posted, but you'll probably have to take out whatever you've got in there now to make room.