There is no performance chip available for the 3G. Anything you see on Ebay, wherever, is a scam.
Start with the basics, Since you already have an intake, get catback exhaust next. You dont need headers. Your Eclipse is a Fed Spec and has pretty good flowing manifolds as it is. You might gain a few from it, but its not worth the cost.
The one thing I HIGHLY recommend is getting a firm front motor mount from
www.deyemeracing.com It only costs between $50-$60, and it is by far the best bang for you buck mod around.