What is your AGP aperture set to? If you have the option of changing it anyway. Try making the setting bigger if it's set small. This will reserve a space in system memory for your VP to store textures and draw them out as it needs them. Too big can be bad, and too small can be bad, and ideas about the "perfect" size change every day it seems. Right now it looks like people are deciding that the "right" setting depends on the game you are playing. As long as you dont set it for more memory than you have available (cant use what you dont have) you shouldnt cause any bad things to happen playing with it.
Right now, honestly, your bottleneck is the CPU, and your FSB speed. The extra RAM will help ease the load on the CPU a bit, as you've noticed from the improvements, but I'm not sure how much of a FPS increase you'll get without increasing the CPU.
Oh, and all RAM up to PC3200 should be backwards compatible with PC2100. Whether your computer can or cannot take advantage of the faster RAM, it will just run at the speed of the slowest stick.