Whoops. I just looked at the manual for the board I'm using. It's a BX board (ECS P6BX-A+) and, apparently, the largest size DIMM it will accept is 128MB. 256MB may work I'm guessing but I'm pretty sure 512MB won't.
Either way, I want PC133 just so I can use it in some other system down the road (i.e. maybe a free upgrade for my dad's 128MB machine)....
Of course, if someone wants to give me a great deal on some PC 100, I'll consider it...
C