ok, Here's some basic definitions:
PC133 = 133mhz bus (SDRAM stick is certified to run at UP TO 133mhz)
PC100 = 100mhz bus (SDRAM stick is certified to run at UP TO 100mhz)
CAS3 & CAS2 (I won't go into detail, but cas2 is faster than cas3, but also more expensive)
A Duron Processor (or AMD Athlon Thunderbird as well) can use either a 100mhz or a 133mhz RAM BUS (in spec). This means it's totally up to you which you run, but strongly reccomend the 133mhz bus as it is much faster. Please note this is the bus speed of the RAM, the processor will always run at 100mhz DDR (200mhz effective) in spec.
Summary: if you have pc133 ram, you can run it at either 100mhz or 133mhz, but pc100 ram will only run at 100mhz (in spec). If you have pc100 already, it will be cheaper to just buy more pc100 and run both the new and the old ram at once at 100mhz. It may be smarter to sell your old pc100 ram, and replace it all with pc133 ram, and run it at 133mhz (faster).
To find what speed RAM you have right now, you can either read the sticks themselves (they will probably have it printed on them), or check your BIOS. If your BIOS says "hstclk" you are running a 100mhz memory bus (meaning you probably have pc100), if it says "hstclk+pci" you have a 133mhz memory bus (meaning you have pc133 memory).
Please note this was written excluding PC66 and lower memory. This is because although technically supported by the VIA chipset, it isn't a reasonable solution.
If you have any other questions, feel free to ask me at either of the addresses below.
_______________________
Scott Newton
Newton_Scott@roberts.edu
AIM: Scootin159