Dual or single channel RAM?

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1

Is dual channel RAM preferable to single channel?

For instance;

G.SKILL Ripjaws Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model F3-12800CL9D-8GBRL
vs

CORSAIR Vengeance LP 8GB 240-Pin DDR3 SDRAM DDR3 1600 Desktop Memory Model CML8GX3M1A1600C10

2

If you can have dual channel memory go for it.  The performance increase isn't massive but it is there.  Paired RAM is also preferable.

3

It depends what you want to do with it.  Generally the only use for 1 stick of ram is so you can buy a second later on.  If you're gaming 8gb will be fine and the 2x4 will be faster but if you are editing and on a budget buying 1x8 will be a better choice since later on you can grab a second 1x8 and throw it into the next channel. The same difference applies between 4x4 and 2x8 and so on.