2 gigs is overkill, 1 gig should suffice for quite a while. 512 mb is about the norm nowadays. It all depends on what your purpose is for the computer. High end games or anything concerning CAD, video editing, photo editing very large high resolution pictures would all be happy with a gig. Pretty much anything else will run good with 512 mb. 1 gb is future proofing your computer and by the time apps need 2 gb of memory it will be time to upgrade your computer with new memory and mother board and all that stuff. A rule I like to use is to look at what a company like Dell is offering in their high end models stock, which I believe is 512 mb, and I double it just to show them bastards who has the better system.
Only get two sticks if your mother board supports Dual Channel memory, other wise it may be cheaper just to get one stick.
-----------------------
Add witty comment here.