CL2 faster than CL2.5, CL2.5 faster than CL3
that's about it
the difference? ok, let's say two computers, one with AthlonXP1800+ with CL2 RAM, one with AthlonXP1900+ with CL2.5 RAM
they would probably perform the same, or almost
see the difference?
it's always recommended to by CL2 memory, to maximize your system's performance, but it depends also on your mobo/chipset.