Id LOVE to find an AF-s 80-200/2.8 for around $500! I paid that for my 80-200/2.8D. About the Sigma, I've seen wonderful examples taken with it, and I'm sure it will live up to your standards. You will also get HSM/II, which is super nice to have as the torque on the 80-200/2.8D is quite noisy when it first engages. I'd look for the DxO charts for both, compare them, and choose from there if image quality is what you're looking for. Anything other than sharpness can be adjusted in PP (or in camera settings, really)..but I can't see anything really being an issue with either. Obviously, the 80-200/2.8 will be worth more in 10 years, or 30 years, or 70 years because it will last that long, but it's really just a personal choice if that extra money will be worth it, or if you would just need another month or so of savings for whatever magnificent lens the come out with next to replace the 70-200s.
I own the 80-200/2.8D and I love it. The picture quality is immaculate and I really don't have anything to complain about besides the weight, but that's only because it is of such high quality (and fully metal). It's really a personal choice. Sigma also offers a 70-200/2.8 HSM OS I believe. I don't know the price point on this, but I'm sure it is quite promising as well. If the 70-200/2.8 HSM OS has as good picture quality as the 80-200/2.8D (It probably comes negligably close), I would sacrifice the extra $200 in 20 years for the HSM and OS today.
Mark