For the money, you cannot beat the JDM ITR manifold. They go for around 400-500 bucks. While some other companies might offer marginal increases (Hytech, Comptech, JUN, TODA, etc...) their manifolds are generally more expensive. I was looking for the old website that had a bunch of comparisons of all sorts of headers (It was a tech article on B-series.net, or a similar site.. not a forum thread), but couldn't find it. To summarize what it said, for all it's worth, the JDM ITR manifold is the best buy for the money, hands down, if you're going to go 2.5". The other manifolds offered one-two, and one case (I believe it was the $1000 Hytech 4-5hp at peak) increases over the ITR. If you're not in california or another emissions controlled state, you can pair it with a 2.5" Type 1 Racing cat, B-pipe, axleback exhaust and (which is my current setup) and it bolts all together with no fuss (except for the extra o2 sensor hole that has to be tapped).
All told, I bought the header, cat, b-pipe and axle back for less than a grand, and saw a noticeable difference.