^^ if your looking for gains on hp/torque.. headers will do that but only if you have had head work done to improve air flow/breathing (ie: port/polish, bigger throttle body, larger exhaust pipes, and refined fuel mapping) if you dont have all of those mentioned (ie: engine/exhaust is stock) headers are pretty much useless.
Shorty headers will provide you with low end grunt (ie: torque) between 2500-3500 rpm, but you will lose in the upper rpm range.
Long downpipe headers will provide you with midrange to upper rpm torque, and thats pretty much where you want it, between 4000-6500rpm.
Hope that helps you Flav in understanding Header performance.
(somebody correct me on the above if i have it all wrong)