Where is the cheapest place to get RAM memory for Laptops?

Im on vista home premium myself, i have 4gig ram installed and it shows me 4 g's in control panel, system and maintenance, system.
Service pack 2 is installed also.
Where did you check to see how much ram is installed?

where you go to my computer it tells you at the bottom has like a bar at the bottom, i think mine is showing less due to the on-board graphics card eating some of the memory up