I would say in this order due to price and performance:
CAI ~5rwhp, I hear they are cheap on ebay
UD pulley ~8rwhp, 99+ about $190 with belt
Duals ~5rwhp, $200 if you get T/O GT duals cheap and pacesetter adapter
Custom SCT chip ~7-12rwhp with 93, $299, better auto trans shifting
Some may want to do gears and t-lok or stop and save up for a supercharger.