I agree. It's worth it if you are doing moderate gaming and a lot of digital photography or scanning and manipulating images right now. You would probably notice "more than a second" difference in those applications.
Otherwise, you will run fine with pc133 for quite a while, and the jump to 2100ddr isn't likely going to be worth the money and effort for you.
I'm running 256mg pc133 on a ecs k7s5a motherboard. I bought because it was cheap, I could still use my pc133 ram, and could upgrade to 2100ddr (it has the slots for it). Well, for about $45 I could get 256mg 2100ddr, but I haven't even been willing to do that because I am doing fine with what I have. I'm just going to wait and skip this jump, and when I'm feeling too out of date, go to 2700ddr (or higher) when I switch mobo's.
Admittedly, I'm a "trailing-edge" technology guy to save money, but I still don't feel too out-dated with the pc133.