depends on how much money u'r willin to spend, and also which route for more power you wanna take. some people go forced induction (NOS, turbo), others go natural.
there are a couple of turbo kits out right now for about 3.5k, the stafford fabrications kit and the DA motorsports kit.
if you go natural, i recommend a new header, port and polishing, high compression ratio pistons with forged internals, new Intake
runners with individual throttle bodies.
just depends on your budget and your needs.