Either one will work fine. The cheaper one is probably adequate. Then again, the other one is twice the capacity and only ten bucks more. Your call.
You would keep the old RAM stick in the motherboard and simply add the new one.
Again:
http://www.crucial.com/store/mpartspecs.aspx?mtbpoid=65B39C24A5CA7304
or
http://www.crucial.com/store/mpartspecs.aspx?mtbpoid=3EE22E4DA5CA7304
Your call.
Also, I doubt you need this, but here is the hardware guide: