Under drive pullies do help make some extra ponies by slowing down the rotation of the accessories i.e. A/C compressor, alternator, etc. But the down side is that the slower rotation of the alternator ultimately kills it. I have used under drive pullies before, along with several of my buddies and we all have had the same problem and we all put our stock pullies back on with no problems since. If you do put them on you can expect your alternator to last maybe 2000 miles.
The only sure way to add HP and torque is to add displacement, put a larger cam in and better flowing heads. You can put just a cam in though and get some pretty noticeable gains. You can put a different manifold on along with larger throttle body, but these will have minimal gains with out adding the other mods to the engine. Also all of these mods will require a custom tune. You can do that by having a PROM custom burned for you or adding an aftermarket ECM like one from Dynamic EFI. I am using the EBL Flash from Dynamic EFI and I love it. It is easy to tune with a laptop and it has self learning capabilities to aid in tuning the base fuel map. It is also easy to install. Some people are intimidated by the re-pinning of the ECM's (if you have a 4L60E trans, you must retain your stock computer to operate your trans), but this is way easier than it seems if you have even basic electrical wiring skills.