Headers are quite a bit more effective than the stock manifolds - about 15 horsepower on most tests I've done. The stock Pontiac manifolds are actually quite restrictive. The Pontiac Ram Air manifolds are much better, but still not as good as tubular headers.
For a street car, the brand and tube length of the header is not that important - the key is to simply get the flow on the exhaust side. The cam you're using really needs some flow on the exhaust side, and any headers will achieve this.
Header installation on a Pontiac is not an enjoyable process. The header flange bolts are difficult to get to due to the angle of the exhaust ports and the tight wrap that the header pipes have to make to miss the crossmember. Make sure you get a set of header bolts with 6-point 3/8" heads so you can get an open-ended wrench on them. If you get bolts with heads bigger than 3/8", you won't be able to turn the bolts, and if you get 12-point bolts, you can't get an open ended wrench on them (which you have to do on the center bolts).
If price is not a problem, the Hooker SuperComp headers are top-notch. There are cheaper ones available, but you may need to do a little grinding on the bolt holes and a little hammer work on the tubes to get them to fit. Also be aware that any header on a GTO will reduce ground clearance a bit.
The ceramic coated headers have a couple of advantages:
1. They stay looking nice much longer than painted headers
2. They run cooler, keeping the heat in the tubes and out of your engine compartment. By keeping more heat inside the header tubes, exhaust gas velocity is maintained, so the headers have better scavanging characteristics.
Finally, make sure your cylinder heads have the end holes drilled and tapped for the header bolts - a lot of Pontiac heads do not have the holes for the end bolts, so you have to drill and tap them.