Dave,
It seems most people here got the most impotant stuff in perspective. You have to practice a lot, the gear will follow.
The biggest reason for people to buy a 625 seems to be fast reloads, due to the enormous charge holes. If you get a 646, the holes are relatively the same size (holes are smaller, but so is the cylinder). If you get a 610, the holes are relatively small(er), so you'll chamfer them to "as big as you like". A 686 will bring speedloaders into the game, plus you'll shoot minor (or work up some funky .357 loads), so I personally would stay away from them and go with a moonclip gun. Just go with whatever you feel is best, or whatever is the cheapest. I know you already load .40, so why not go with a 610 or 646? With practice, reloads will be just as fast.
You say you don't know a lot about revolvers. Well, there's not that much you should know to be competitive. You just have to know YOUR revolver. And practice like a maniac.
Good luck pickin' one. And remember: Buying a new gun is fun!