from a header or a bolt on muffler ??

The header for sure, bolt on muffler probably won't give you much of anything (you might gain 4-5 HP from a good cat-back exhaust)

Originally posted by 6'5 290lbs in a G20
from a header or a bolt on muffler ??

Many of your newbie questions are answered quite well and often in great detail either here:

http://www.se-r-list.org/archives/

or here:

http://www.se-r.net

Headers for sure...