hypertech power programmer III

I am looking at purchasing the hypertech power programmer III for my 96 K1500 5.7. What benefits in horsepower, torque, and gas mileage can I expect to see from using it?

Actually, for around $300 plus, you will benefit much more with mechanical mods first. K&N, free-flowing intake, free-flowing exhaust, headers, throttle body spacer, MAF sensor, etc. The programmer is nice in that you can change your computer settings and all, but you will not notice any really noticeable gains in power or mileage.

What are the more common exhaust upgrades done to to get more power out of the vortec? What brand of headers, type of exhaust, etc.

Different exhausts produce different types of power. Therefore, your decision is based on what type of power you want. I.E. Top end, low end, all-around, loud, quiet, etc. 2.25 or 2.5 inch pipes will give good low and high end power. I prefer the 2.5, but there is not a HUGE amount of difference.
With headers, stick with the 1.5 or 1&5/8 inch primaries. Go to summitracing.com and type in part # SUM-G9016; they are good headers. I put some on my truck, and I have had no problems. Get good gaskets, however. Copper or the like. Any more questions, just ask.

What will a complete exhaust system cost including headers? Can an exhaust kit be purchased or does it have to be custom bent at a shop? Also will this interfere with any emission equipment, sensors, etc. and can I expect a change in gas mileage?

As you can see in Summit Racing, headers can be found for under $200. As far as an exhaust, get yourself some magazines to look at if you want a name brand exhaust. Good magazines for these pickups include: Summit, LMC truck (lmctruck.com, free magazine, numerous performance parts), Jegs, or anything like that. I don't know prices, so you would have to do some research. However, if you want a true dual exhaust from headers back, look into a nearby muffler shop. I have a friend that had a complete exhaust, including mufflers, installed for right around $200. It is custom and stainless steel. You can't get much better than that. But if you want to stay smog legal, you are somewhat limited to shorty headers and a new muffler (roughly).