What do I need to know about RAM?

no, you can mix whatever speeds you want (512 with a gig, gig with 2 gig, etc. . . ) you just need to make sure you get the right type of ram (ddr ram, ddr2, sdram, etc)
check out crucial.com for more information

that’s a simple answer..

It depends on the machine. Some require matching pairs of chips, some don’t. It’s too much for anyone to really know, so trust the online configurators, like the one linked to above.

Thanks, this is pretty much what I was expecting. I already order most of my memory of Crucial, just never bothered to mix chips or speed before and wondered.