Do a compression and leakdown test. Make sure the engine is healthy.

After that, a good Cold Air/Tuned Length Intake, with a good header (DC w/2.5" collector mod or a Type-R), a good high flow cat, and a 2.5" exhaust. If you pick good components and research the parts (aka, not on here), you can pick up a good 10whp, atleast. After that, you can get a chipped ECU and do some tuning and maybe pick up more power. A decent set of cams (Crower makes popular cams for the LS, but get cam gears too) and a proper tune and you'll be good. Cams, I/H/E, a good intake manifold (matched to your set-up) and PROPERLY sized throttle body, and a proper tune, you can probably pick up a good 20whp+. You'll certainly feel that through the entire RPM range.

You're not gonna see huge power gains from bolt-ons, but apart from swapping, boosting it, spraying it, or doing a proper NA/Non-vtec build, that's about as good as it gets. But the key to making good power is choosing parts that compliment each other and getting a GOOD tune.

If you want to discuss build theory and shit, feel free to PM me.