like _dk_ said, the best HP gain for $$$ value is serious weight reduction. Don't know how serious you are about spicing up your ride but those stock seats weigh a ton, so does the factory carpet and A/C system (if you have it).
Whatever you do, DO NOT get a K&N cone filter. It's been proven many, many times that the cone filter only sucks hot air in from the engine bay. Dyno results have proven time&time again that the best filter is a K&N drop-in type with the stock airbox.
My YouTube video
-- RANDY SPARRE --