An engine works like a pump. The better you can get air in and then let out, the better and more efficient it will be, thus more performance...but it is limited for amount of gain. By upgrading your exhaust and air intake (drawing in cool air away from the engine heat and using a quality filter) you will gain some performance, and maybe better fuel mileage (if you can keep your foot off the gas pedal )
Start with these two mod's, then if you still want more performance you will be looking at the heads and internal parts.