Depends on you motherboard, chipset and which CPU you have. If you have a Celeron which only uses a 66Mhz FSB and you aren't overclocking then PC100 will be fine. However, I recommend going with PC133 which is only a few $$ more and in case you wnat to upgrade in the future. Memory prices are pretty cheap right now. Crucial has 128MB PC133 CAS2 SDRAM for under $100 if you order off of thier website.
As for CAS2 vs CAS3, CAS2 is slightly faster. I should also point out that if you mix different speeds, you can only go as fast as your slowest part.