you can use any mb size of ram you want ussually its best to try to match the speed of the ram 100,133,200,266,333,400 ect
just beshure its the same type of ram as in ddr , sdram, ect
you can mix a 100 with a 133 speed . but it will run at the slower speed .
This is for most pc's but id check in your main board requirements just in case.
also if your running 98se as a OS it can use only 512mb's of ram anything over is a waste.