I run 6 accounts each on two machines (2011 intel socket asus sabertooth x79) and 4 accounts each on 4 old 2 core amd machines.

I have 6 1920 X 1200 monitors. I don't tile but can see 6 out of my 28 accounts in 1920 X 1200.

Others will advise different but I would always go with 1compter/1monitor/1videocard and scale up.

I very sure that 2 $1000 computers will out perform 1 $2000 computer.