I'd agree that both are overpriced, especially considering their age (circa early to mid '90s). The original Sigma 28-70mm f/2.8 is known particularly known for lacking sharpness. The original 70-200mm f/2.8 APO is also known for very slow focus. There's also a good chance one or both would need to be re-chipped to work properly with digital bodies.
I picked up a 70-200mm f/2.8 EX D HSM for $500 and 28-70mm f/.8 EX D DF for $165, both here at the Cafe. If you look around you can do much better than these from craigslist.
I agree as well. I got a 70-200 f/2.8 HSM (the non-DG version, same as the one that Stephen has) for $500, and I'm quite happy with it. AF isn't quite as fast as Nikon's AF-S, but it's really quiet, and definitely faster than screw-driven lenses. If you're going to spend $425 on a lens that old, you might as well spend the extra $75 and get something that is worth the price.