selecting RAM

The difference between the sticks you listed is speed. This is not always the most important factor in performance.

I recommend reading Linderman's sticky.

http://www.techsupportforum.com/f210/important-before-you-buy-memory-163105.html

Also, are you planning to upgrade your existing system? If so please list complete system specs. This will help to determine the best course of action.

Doubtful that you would see a dramatic difference if you chose DDR3 over DDR2 and it is specific to the board. If you have a DDR2 board you cannot use DDR3 ram.

If you are really considering a new system build then I suggest you solve this thread and start a new one. Just ask questions re: best build within your budget. List what the pc will mainly be used for , i.e. video editing, gaming, general home use, any work related applications. List your existing components. You may be able to reuse some. Lots of people on this forum would be happy to point you in the right direction.