Well, I'm taking that the fsb is only running at 100mhz so while you can put pc133 in the system, sellling your pc100 for another stick of pc133 woudl be useless, since it's not going to get anymore performance above 100mhz unless you o/c. If you care about things such as cas latency depending on how old the pc100 is it may only have a latency of 3, but I still would find that rediculous to buy new ram for a change in the cas2 latency alone.