oh, OK. PC2700 CL2.5 is better than PC2100 CL2. Here's why: The Column Address Strobe only is a factor on the initial access of that memory column. Everything else is fetched at the same clock sync regardless of the CAS level. So it only helps out a small percentage of time.
PC2700 (166Mhz FSB) vs PC2100 (133Mhz FSB) is ALWAYS a factor. i.e. the bus is constanly running at 166mhz. So that makes a bigger overall impact.
I hope you can understand my ham-fisted explanation. Others might be more elegant.
<i>It's always the one thing you never suspected.</i>