yes you can mix them as long as there the same...i.e SDRAM, DDR, or RDRAM. You are only as fast as your weakest link so if you ahve PC1600 and PC2100 you have to run your memory at the hgihest speed the PC1600 can go (usualy the PC1600 will be the slowest however if you bought noname PC2100 but good PC1600 then your gonna be as slow as the PC2100...) same thing goes for PC2700 and PC2100...basicly find out which stick is the worst and that's as high as all of em combined can go.