If you want to have 2 GB for Photoshop, video editing, Battlefield 2, Quake 4 etc. and want to overclock to high memory frequencies - the G.SKILL F1-4000USU2-2GBHZ PC4000 are the way to go. It does not perform like memory with Samsung TCCD, which usually runs up to 290 MHz at 2.5-4-3-X, but TCCD is only available on 512 MB sticks.
Having 2 GB of memory means putting and end to stuttering while loading and a smoother game play experience.