Doublestack .45, huh? I seem to remember paraordanance had an unbelievable deal this month for members of the military, or former members. Might be worth a look.

I also have a PT 99...great starter gun...I will never sell it!!
But my point here is, I looked at lots of guns (handguns) and tried out quite a few, (kimbers, S and W, colts, but I came to the Glock...if your looking for a never fail gun...get a glock...the trigger takes some getting used to( safety action trigger)but I fired over a thousand rounds through it, of some of the nastiest, diry ammo I could find, and after 1200 or so rounds... without cleaning ,it finally got 1 spent round (brass) caught in the ejector port. So for my $540.00 I chose this one (glock 36) and have never been happier.
Out of the box, it is the straightest shooting weapon ever...BAR NONE!! There are bigger glocks(capacity wise) but my 7 shot acp.45 will take you down in 2 shots!!...ah yes, the double tap...OTJ

I just purchased a new sidearm a few months ago. It won 2007 handgun of the year. It's the Smith

SA-XD in 45 ACP if it fits, else a 1911.

There are some great guns out there. I would rather buy a used Kimber than a lot of cheaper guns new. But my best advice is to stick with what you are familiar with. My second best advice is to buy one of everything. Not very realistic, but fun to think of! After all, shopping for a handgun is like the thrill of the hunt. The actual purchase can be a bit of a letdown.

At the $500 range consider the Baby Eagle...Made in Israel and Combat tested----DAILY-----

You won't find any decent double stack 45's in the $500 range. (or even at double that)

The Glock 23 is an outstanding performer - Trigger takes getting used to but THEY ALWAYS WORK and they carry 14 1 in 40 cal.

I prefer to carry a 1911 but I live in kalifornia, can't get a CCW and I have to decide if want to have a custom 1911 or a Glock confiscated. The Glock always works and I can go home and get another one!

Springfield XD45 (ACP, not GAP) sounds like it would fit the bill for you.

You are not going to get a doublestack 45 for $500. There are mass produced hi cap .45's, Para and HK come to mind, but the prices for the Para's run around $900 and the HK is over $2,000. Although at one time in my life I despised Glocks, after it became my service weapon, I have nothing but praise for them. The Glock 21 has a 12 round magazine and you could pick one up for about $600. They may not shoot as well as 1911 style weapon, but they are simple, reliable, and shoot well enough for most people

I'm buying a Beretta PX-4 Storm 9mm version. Super gun, newer model but a great reputation.

In the $500 range
Springfield XD
Glock-any pick
Smith and Wesson M

Even if you're thinking new, you don't have to go new. Some reliable .45's with multi-stack mags are worth picking up used. I'd stick with Glock, H

A little more than you say you are wanting to spend, but IMO, worth it... the Sig P229R DAK:

http://www.sigsauer.com/Products/ShowCatalogProductDetails.aspx?categoryid=8

Try a Sig Sauer .40 SW. Great gun with more oomph than 9mm but easier to shoot well than a .45.

My mates refer to me as a Glock snob. I've fired just about handgun know to man, and I've not handled anything that I would replace my Glocks with. Firearm purchases are a personal issue. Buy what you feel most comfortable with - whatever best fits your operation, you hand and your budget is the right handgun for you. For me, that is a Glock, combat plastic at its finest. Good luck and stay safe.

I carry a Sig Sauer P239 in .357 Sig, I like it. Capacity of 7 1. Doesn't come with a safety, you gotta supply that yourself, but also no fumbling for it if I ever have to draw.

Glock 21, S

doublestack nine, like the CZ pistols, are very nice and right in your price range. doublestack 45's are bulky, with the Springfield XD being the slimmest i know of.

P-220

have you thought about a bersa .40