Your dell can't take advantage of 1600 memory, so I would get 1333. I recommend you pull one stick and get the same brand if possible. It may be hynix, samsung, or crucial. Crucial guarantees it's ram to work if you buy from their website, but you'll pay a little more. This could save you return costs and restocking fees if your ram won't post. Don't mix ram. Either fill two or four ram slots with the same brand/speed of ddr3, and sell the leftover single stick. You can leave two of the sticks installed and get two more if they are identical. Dell wants you to buy the ram from them, so their boards are very specific about the ram specs. Hard to tell if some brands will work, so the same is your safest choice.