1. I use ypool.net
2. if you have the hash power, it seems to be pretty profitable at the moment
3. I believe difficulty is gonna drop next cycle cause at current difficulty AWS/Cloud mining has become unprofitable which caused a huge col/m drop
4. Ram bandwidth seems to be the deciding factor. With my 2xX5560 with Tripple channel RAM I get about 380 col/m under CentOS. That drops to about 315 with single channel RAM config.
5. Where's 5. It's not in your list
6. Again, memory bandwidth seems to be the deciding factor. But more speed always a good thing.