the motherboard may take DDR2700, but the XP 1800 wont run at that bus speed...this is still fine as the memory should underclock to the bus speed of the cpu.
with the XP line, AMD went to a "double speed" bus architecture; 2 bus per processor. thats why I said 133 FSB and Crucial is saying 266, Crucial is giving you the "double speed" value for the 133 FSB.
DDR also performs best when it is installed in pairs like old-school sims(but with sims, you HAD to install in pairs, with DDR you dont have to)
if your original DDR memory is 2100 then your newer memory wont sync up for best performance because of differing speeds, CAL settings, etc...this performance increase isnt always very noticeable though...it is noticeable under heavy video rendering and 3d modeling which is much more ram intensive than audio processing is