dumb question probably, should i get an exhaust before header?

the thing is, i really want a full cat exhaust, but i also want a header, but i don't have enough money for both right now, can i get the exhaust and then add on the header later (well of course i can , but) would it be harder or more costly to install if i did that... umm.. yeah... and how much hp do you think a header will get anyway.. or a whole I/H/E setup