I've installed a K&N air filter and magnaflow stainless steel muffler on my Inf. G20, can someone tell me what other performence parts/upgrades I can look into. I'm not looking for a air intake though.

why are you not looking to do an intake? the stock airbox and intake tube can be one of the biggest restrictions.
stillen would have parts for your car.
i would do the intake, header, high-flow cat, short shifter, struts and springs, and a brake upgrade.
if your muffler is on your stock pipe, i would take it to a muffler shop and have them remove it and run 2.25" or 2.5" from the cat back.
seeing as how the engine is the sr20de, you could consider turboing it, or you could swap out to the sr20det from the bluebird or pulsar gti-r