James,
Welcome! I've had a lot of experience being a big dude on a lot of bikes, and I'll be happy to share those. For your goal size you are going to need something with modifications, but you should be able to make it happen. I weigh 350 and ride a Surly Long Haul Trucker, which I started riding when I weighed around 388 last year (yeah, it's been a slow year ). The only thing I've done to it to accommodate my weight is to have the stock rear wheel rebuilt by hand by a great wheelbuilder. Neil (The Historian) is correct, I'm a HUGE (in certain ways ) fan of the Specialized hardrock, I started riding one at around 420lbs, no modifications at all. The stock wheel lasted for over 2,000 miles before developing cracks, and I had it replaced just recently with something even tougher.

Just my opinon, but I would say for 500lbs you would be able to ride either of these two bikes, assuming you are losing weight consistently and quickly on them. One thing that is going to get you is wheels - at that weight you'll kill 'em. Either bike with a tandem wheelset (40-48 spokes) will do you well, and be sure to run wide tires on in. The Surly comes with 700x37's, which would be fine for you, and the Hardrock's 26x2.1 knobbies would work, but I'd go with something like a Schwalbe Marathon in 26x2 or Big Apple in 26x2.

One last note: I've been there, well close. 567 was my top number, and it was a different world. It sucked, simply. 350 ain't great, but my life has changed dramatically for the better. Don't stop -even for a second - because if you do it's a slippery slope with a jetpack on your back. You've gotta want it, and want it BADLY. I've been there, done that, written the book, etc. My brother lost a bunch of weight - like, he probably went from 450 down to the 200's, but he slipped up. Now he's back up there, and I can tell it isn't a good place.

Keep with it - we're all around here, and a good group to boot.