Ram, Whats The Difference? Options

[fela]

Hi im not too clued up on the internals of computers so forgive me if my question is really obvious.

I own a G5 duel 2 ghz and i want to expand the memory from the original 512 DDR2 by an extra 2 gigs.
When buying the new memory will it have to be DDR2 in order to work with the existing 512? and also does it matter what make i get like Kingston or GeIL for instance.

Thanks