well i dunno about needing nitrous to make a civic fast - or at least quick. You can make a civic much more fun to drive (remember, not all of us are after 1/4 mile times) with some simple mods, but you have to do some homework and not just throw parts at it hoping something will happen.
As for your gasket, find out what you need from whoever made the header, then get it from Mr. Gasket. (
http://go.mrgasket.com/Brands.aspx?BrandID=1)
You'll need to compliment that header with a new cat and a new performance exhaust system before it does you any good (and that's assuming the header is a decent one). You'll also need to improve the intake side.
There are several Civic/CRX forums out there that can help you find more power. . . .