air nailer

   A 2" Brad Nailer would probably be your best bet. Porter Cable makes a fairly inexpensive model, although I prefer the Stanley Bostitch or Senco. The 2" guns can shoot a brad as short as 5/8" - a good size range for most of your needs.

  I'd suggest buying a nice 18v cordless drill for building boxes, etc., as nails won't hold a sub enclosure together as well as screws over the long term. The drill has so many uses too. I love the DeWalt 18v drill ($190). Great investment. The brad gun would be useful for tacking a box together while you strengthen all your joints with screws.

    Dave

   Great tools @ www.woodworkerswarehouse.com

   Inexpensive tools @ www.harborfreight.com

Check this out it’s a cordless nailer just bought one of these very happy with it. Much easier to use than my old air nailer a lot lighter and smaller, much easier to get into those smaller spaces. Defiantly worth the money.  However the price is a little steep Got mine off eBay for about $300.

http://www.paslode.com/products/tool_catalog/IMCT.html

An airnailer will do just fine for assembling boxes.  Let's not forget that whether we are using nails or screws to assemble boxes, these are used as TEMPORARY fasteners while the glue sets and should not be relied upon as a "Key" that holds the box together. 

If you are looking for a good cheap nail gun, airy makes a kit with a brad nailer and a staple gun for about $120. Even though it is cheap, it is still very reliable. Oiled daily, it has survived over 1 and a half years full service in a cabinet shop, often being used when the porter cable and the stanley jam up. If you are looking for a basic no frills attached gun, this is the way to go.