these are 2 different types of RAM. PC133 is 168 pin, PC2100 is 184 pin. big difference. some mobos take both i believe, but find out what you have first. you can take out your current mem stick and look for a tag saying what it is or look up the part number online for specs.
ASUS P5P800 - P4 530 3.0GHz
2x256MB PC3200 2-2-2-5 - WD 74GB Raptor
PNY FX5200 128MB/256bit