I have a HP Omnibook xe4500 laptop (technology code KB; 1.7MHz Pentium 4M, 30GB disk, ATI Mobility 32 MB, ...). I bought it approx. 2.5 years ago. It came with 256 MB RAM installed (Samsung 256MB DDR PC2100 CL2.5). I want to upgrade it (one slot is free). The manual says that I should buy PC2100 DDR266 or higher RAM. Can I buy an additional 512 MB? Will it work if the chips are not of the same speed (let's say if the new one is 400MHz) and not of the same capacity?