in respect to heads, stage 1 would be mildly ported and polished to give a slight increase in bhp and jumping to a stage 3 would probably include bigger valves and guides and heavily ported and polished inlet/outlet ports to give a much larger increase in bhp, although different tuners have theire own different specs for each stage. the same sort of rule applies to turbos,basically a stage 3 head or turbo would produce more
power than a stage 1 or 2.
hope this helps.