Can I use two different size memory modules?

I upgraded the memory on my laptop from 1 256mb card to a 512mb card in slot a and put the 256mb card in slot b. I have a toshiba A45 S120, do i need to take the 256 card out for it to work at 512 pace, or does it matter that i have two different size cards in the slots?

Toshiba Satellite A45-S120 laptop has two memory slots available for upgrades. This model will accept up to 2GB. You should use PC2100 DDR266 memory modules. You do not have to remove the 256 memory module. It is perfectly fine if you use two different size memory modules in both slots. If you use 512MB in slot A and 256MB in slot B, the laptop should recognize 768MB installed. You can use 128MB, 256MB, 512MB or 1GB modules in each slot. Another story would be with the memory speed. Your laptop is designed for PC2100 (266MHz). Most likely you can use PC2700 (333MHz) module but it will clock down to 266MHz and you memory card will run at PC2100 pace (266MGz).