Techically it wont be ‘faster’ just youll have more!
I would recommend buying a stick of DDR400 (PC3200) as if and when future upgrades come you will have the faster RAM to utilise!
Faster? No. Now, PC3200 will run fine at PC2100 speeds, simply because it’s only running at a slower speed than it’s rated for. However, the PC2100 won’t run at PC3200 speeds because it’s not made to do so. If it does, you’re very lucky or you got a mis-labled stick somehow.
But, yes, given that you’ll be paying about the same price for PC3200 and PC2100, I’d get the PC3200 instead. PC2100 only real issue is that it’s not the standard like it used to be, whereas PC3200 is.