There is a difference, not for the CPU, but for the lousy Integrated Chipset
I have a Toshiba M35X with Celeron 1400 (running @100mhz FSB). It came with PC2700 (333mhz) DDR
So I put in a pair of DDR2100, thinking that it won't matters since my CPU is only running @100fsb.
WRONG - it crash...