RE: 45 auto????????????
If you start looking at a .45, then I suggest you take a look at the Springfield XD45 ACP. I've got an XD9 (9mm) and I love it. The XD45 is new and may be a little hard to find, however. The XD also comes in .40 cal if you decide on that.
I bought a 9mm for two reasons: 1) my wife was more comfortable shooting it over a .40, and 2) 9mm ammo is cheap (about 20-25% less than .40 and 50% less than .45) and that means I can practice more often.
You'll get a wide range of opinions on the suitability of various calibers for personal defense, but I think it all boils down to being able to practice enough to hit at what you're aiming and selecting the right ammo.