Follow the links in this thread for some good info. I also talked to guys that were into spearfishing already, and talked to folks at my LDS. My LDS was nice enough to let me try my gun out in the water before committing to anything, so that was very nice.
Lots of folks will try to talk you into buying the most expensive gun you can find. I looked at the rigs that were actually being used by other divers, and found that many of them used A.B. Biller guns in the 42"-54" range. Next in popularity are the Riffe guns, too rich for my blood at the moment, but perhaps at some later date. (the Riffe guns ARE very nice btw)
I went for the 48" Biller. If the other guns are more accurate than the Biller, then I'll never need another gun. The thing shoots like a laser (I'm used to shooting pistols, so that may have helped). Of course I was shooting stationary fish-sized targets, and current was not an issue, but I'm hoping that some of my practice success translates into fish this weekend (I'm going on a spearfishing charter). I'll be sure to report my victories and failures after the trip.
ps: Whatever gun you get, I found it very helpful to practice reloading in the water. First, when you are concentrating on the gun, your buoyancy control will suffer. Try to stay at a constant depth while reloading, so you don't blow your ears, or ascend too quickly. Also, I learned the hard way that you can easily mess up your band and lines underwater. Practice proper placement of band, line and shaft so that your gun will work properly when it comes time to pull the trigger. Finally, don't forget to mind your safety, and make sure it stays on until you are ready to shoot something.