4 GB RAM card has not came out yet and if it did, I'm 100 percent sure he did not get it for 15 bucks.
Kingston I believe makes the best RAM you can get. It all depends on what you are looking for. You say you need 512 more but I would get a 1 or 2 gig stick. 512 MB goes from about 15 to 20 buck, 1 gigs go from maybe 35 or 50 and 2 gigs go form about 99 to 150. Not too sure about the prices because I've seen many different ones. But what are you looking for, DDR1, DDR2 or DDR3 RAM because I have a 512 DDR2 RAM card I'm not using I can give to you. If not then you can go into Best Buy or Staples to see what they have. But I recommend going with at least 3 or 4 gigs of RAM if your going to play games and maybe updating your graphics card. hope this helps.
Oh, and get the desktop PC RAM cards and not the notebook RAM card if you have a desktop computer.