I have Dell Inspiron n5110 (Windows 7 64bit) with 4gb ddr3 PC3-10600 (667MHz) ram but I need it to be faster for projects so I decided to upgrate the RAM. Is it better to put another 4gb ddr3 ram stick to my empty slot or to buy 2x4gb and change my existing one? Is there going to be a huge difference?
Can I add this one to my laptop (Corsair 4GB (1x4GB) DDR3 1333MHz (PC3 10666)?
I would update both chips since it would be quicker RAM. I've seen some comparability issues with some RAM speeds and boards but that Corsair should work. Here's a link to Crucial RAM which is compatible. Also, if you're looking for over-all speed upgrading to a SSD would be a great addition as well. I've provided the RAM and SSD links below:
Your current ram is 1333 not 667, you confused actual speed vs effective speed so it won't be quicker ram. Laptops and desktops use different sized ram sticks so are not compatible. You'd probably be fine with just adding 1 stick as long as the specs are the same. You can check the specs from cpuz.