You're not going to gain that much with an underdrive pulley. Keep in mind that a 5% gain on an engine with about 100 horsepower stock is about 5 horsepower. The cam is a bit more work than you would think, too. I would recommend saving your money and getting an adjustable cam pulley. This will allow you to tweak the powerband just like you would be doing with the cam, but without as much work. I had advanced timing on my '95 Spirit and it made a HUGE difference in low-end power (based off the very casual observation of how long the tires squealed when floored). However, compression jumped up enough that I was endangering the headgasket (on the order of 175-180 psi per cylinder). That was one tooth, or 7 degrees, advance. Most cam sprockets go up to about 5 degrees, and you can play with the timing without really having to dig into the engine. All told, you'll spend less money this way, too.
I currently run a K&N filter (helps a little, but the performance gain and dirt flow through the filter is pretty highly debated both here and elsewhere) and a 2.25" exhaust to a Super Turbo (a royal PITA to do on my car, since I DIY'ed it and had no previous experience with exhaust. I believe Dynomax has a couple of pre-fabbed parts for the P-bodies in this size, though). Far and away the biggest jump came from the exhaust; I dropped 2 seconds off my 0-60 and now average over 25mpg mixed.
I agree with Bob on the lack of potential gains without head work from the cam swap. You'll get something, but it is pretty much a waste without at least a port job.