PC-100 and PC-133 are both the same RAM type, but run at different speeds depending on your system bus. Your G4/500 has a 100 MHz system bus, so you'd need PC-100 SDRAM - however, you could use PC-133 SDRAM, but it'd run at PC-100 standards. PC-133 SDRAM is designed for 133 MHz system bus Macs.
Your G4 can hold 2 GB of RAM, but the MacOS can only use 1.5 GB of RAM, so don't add 4 x 512 MB DIMMS. Just some useful advice.
I use PC-133 RAM on my Sawtooth because I may someday get a newer G4, and I want the flexibility of being able to switch the RAM. Mixing speeds seems to work OK. I used to run one each of PC-66, PC-100 and PC-133 in the 3 slots of my beige G3 (66 MHz bus) with no apparent ill effects.
Just be careful that
a) your firmware is up to date 4.2.8
b) you can return the RAM as some modules are not recognised especially single sided.
Generally pc133 is fine.
PC100 222 ( CL2 ) will actually give a slight speed bump but being forward mobile probably outweighs that very small benefit.