Definitely sounds like a finish nailer is what you're looking for. For a true pneumatic, I would suggest Senco. A bit more expensive than most of the others, but worth it, IMHO. They seem to be best at driving the nail to the proper depth with the most consistency and with a minimal amount of adjusting. They also have a wide range of models to accommodate 15, 16 and 18 gauge finish nails. It's a pretty simple thing to go to Home Depot, look at the different gauge finish nails for yourself, and decide which is most appropriate for the size project you have in mind.
I'm assuming you already have a suitable air compressor, hoses, fittings, etc. If not, then never too sharp's recommendation of a Paslode is a very good one. I've been using one of their straight magazine 16 gauge finish nailers for over 5 years now and couldn't live without it. For things like crown moulding, baseboard and casing you just can't beat it. Having no compressors, extension cords and hoses to drag around is a real plus in that situation. If you'll only be using it in one spot, however, like in a workshop, then the advantage here is obviously not as great. The Paslode gun itself will cost close to double what a good pneumatic will run you, but if it saves you from having to buy the compressor and hoses, or if you'll be using it in many different locations, then it's definitely worth it. These are available in 16 and 18 gauge varieties.
There is one minor downside to the Paslode when it comes to fine finish work. The amount of pressure it takes to depress the nose piece (the gun's primary safety feature) is greater with the Paslode - as compared to a pneumatic gun - because it is this action that also mechanically delivers a metered injection of butane into the nailer's combustion chamber. This can sometimes leads to frustration when you're working alone trying to nail together wood pieces that have glue applied to them - especially larger, awkward pieces. The glue tends to make the pieces slippery to begin with, and the extra force required to depress the nosepiece sometimes exacerbates the problem. And yes, you'll want to be using glue for projects like cabinets and shelving. It's not a huge issue in my estimation, but should at least be mentioned.