If the chipset is LX you wont be able to do much. the fastest chip the LX chipset can take is a celeron 533
LX's max FSB is 66, but i think some of them could do 75 or 83, for overclocking. (but being gateway, i doubt it.)
now if it's BX, it can take at least a PIII 600 katmai, and at best a celeron 1.1 Cumine or PIII 1.0
so.. basically.. find out what chipset it is

