One of the more popular questions. The other is 4 wide or 5 wide? (But I digress)
I use 6 inches of memory foam (Wally World) in my 6 wide and I put a thick -30 degree Slumberjack Woodsman sleeping bag on top of that. I probably have 7.5 inches of mattress total.
My advise is to test your mattress in every position that you sleep in. The most important thing is not to bottom out - that's a horrible feeling on your hip etc. in the morning.
Scott G.