Mixing RAM in a laptop?

I have a crappy old gateway MX6959 with 1GB RAM in it.

Is it going to harm the computer if I throw in a 512MB stick in it?

And yes, I have built desktops, I have taken plenty of IT classes, so your higher lingo will not scare me ;) Feel free to respond in your jargon.

Why is it that you'd want to do that? (games, programs, ..?)

I shouldn't do it if it's so old as you say. (except if I'd wanna be able to run more games or programs on it and only have to little RAM)

ive seen prebuilts with mixed memory

Well its probably running in dual channel now, but adding a stick, will prolly mean its not.