I had a stick of 4 gig RAM just go bad in my system built around an ASUS P8P67 board. I was using GSkill Ripjaw 10666 (2x4) Ram. What type of RAM should I get for this mobo concerning speed (10666, etc.) ??
Also, since one of my sticks went bad I'm running in single channel mode on a 4 gig stick obviously, is this much worse than Dual channel mode? Is it worth it to upgrade back to 8 gigs?
The difference between single-channel and dual-channel is a few percent. 1600 (12800) is the price/performance sweet spot for Sandy Bridge systems.
As far as your last question: I don't know ... is it? Do you notice a difference with only one stick? Do programs/games load just as fast as they did before, or is it slower? I know I noticed immediately when I went from 8GB to 4GB, but depending on the programs/games you use every day, you may not.
G.Skill has a lifetime warranty. Contact them and RMA the memory. You should at least get back to the 8GB you already paid for.