You only have a few choices:
First of all, definitely change the rear gear. While it doesn't add horsepower, the engine will be able to get to its power band much quicker. It makes a world of difference.
As far as actual power, most popular and pretty cost effective is swapping to 99+ intake, heads and cams. This will put you in the 270ish hp range (at the crank) due to a small bump in compression over a factory 99-04 engine.
If that doesn't interest you, next most cost effective choice is nitrous. The downside is you have to open the bottle when you want to use it, as well as refill it, make sure it stays a certain temp., etc. It IS the biggest bang for the buck, though. Assuming your engine and fuel system is in good working order, the car could handle a 100 wet shot all day long.
If that's not your cup of tea either, look at basic Vortech kits.