I currently have tables of four. I find it easier to wander around and discreetly help the less able (and *** the lazy!). Rows makes this quite difficult - I find I'm constantly squeezing past students - but that might be due to the size of our rooms. I always use a seating plan. It helps me to get to know the names of students I haven't taught. I usually start it off fairly randomly - mixed ability tables, girls next to boys etc and then adapt it as the friendship groups etc begin to rear their ugly heads. In my experience, alphabetical order just means that some students are sat with each other in every subject - sometimes a good thing, but, as you say, more often than not a bad one!