fsb =front side bus speed.....so when you see a CPU you might find for example 1.33ghz 266...that means 266 fsb and can run double pumped PC-133 sdram(PC2100 ddram--double data ram)....or you might see a 1 ghz 200fsb which uses PC-100 sdram(or pc-1600 ddram)...these chips can use variations of these, but that's what they are optimized for.....the only other factor in determining what ram to use is your motherboard and what it's chipset supports...in your case you can install up to a max of 786mb of pc-100 sdram i.e 3 x 256mb sticks, or 1 512mb+ 128mb sticks(you only have 3 slots for memory sticks)....you can also install a 1ghz cpu w/ 200fsb max, or any athlon below that i.e athlon 800mhz or whatever