Probably the most comprehensive site for RSX information is ClubRSX, once you sift through all the crap. Their forums don't work with Netscape 4.7x, however.
I go there for RSX-specific information, but Team-Integra is my choice for community and general helpfulness. If you do perform any upgrades to your Type-S, please take lots of photos and consider composing an article describing your project to share with your fellow Team-Integra members.
DC sports should be coming out with a header soon too. I know that u can get the JDM headers at clubrsx.com There are also quite a few other exhausts out there such as A'Pexi, Magnaflow, Spoon and others, if you go to clubrsx.com and go to the online store they have just about everything out there.
pay attention to hytech. john is creating a turbocharger and a supercharge along with NA goodies for the rsx. when he finishs everything its gonna be interesting to see if the numbers hes claiming will live up to expectations. im sure they will