You can actually add another 256, or a 512 or 1 Gig there depending on the type of RAM you wish to install.

If the rams are identical like all 1-sided or all 2-sided modules, they can work harmoniously. You can't just mix a 1-sided ram with a 2-sided ram.

But the safest way for you to do is simply bring your laptop to the store where you wish to buy the ram upgrade. There, you can readily test the compatible ram and buy it.