Basically it has to do with how many clock cycles the module requires to respond.
CAS2 is thus faster than CAS3, & right now since memory's so cheap pick up as much CAS2 as you can afford. 256 MB is probably the sweet spot right now.
I personally have a 128 MB Mosel Vitalic REV2 stick, with another matching stick on the way. Good stuff, does 143 CAS2 with ease, frequently up to 150 or 160. Mushkin has it for $140 right now, or John over in FS/FT has it for $119 if he has any left.
Viper GTS