I find silver rather regularly, as well as gold jewelry (unfortunately the latter is usually just cheaper kids stuff because of where I find them). As far as depth goes, I found a silver dime at what I would've considered the depth limit of around 8 or 9 inches, using the Sniper coil on the 250. This Florida sand/soil helps a lot, though, so I seem to get pretty deep no matter what.
And as far as research goes, I research a lot, but keep in mind also that those older places have no doubt usually seen other detectorists over the past couple of decades whether you may think they haven't or not. One of my favorite old home sites that I like to go to had a fellow MD'er the other day when I drove up to the lot. I've never seen anyone detect this site before, and I find stuff almost everytime I go, and so was he. Now I just need to get my weedeater out there so that I can get into the scrubby brush area around those big old trees! I've had my share of rattlesnakes and water moccasins to last for a while, thank you very much

And also remember, that those old sights could very well have had relic hunters at them even before they were there with metal detectors (I used to do the same thing back when I was in my younger teens when looking for Civil War stuff and didn't have a MD.
Never give up, at least your always going to be finding something more than you would be if you sat at home just thinking about it or watching TV! And you will eventually find something worthwhile, especially if you do research and go off the beaten path every once in a while. Some of my best hunts have been in the woods and fields, where you'd not expect to find anything!
Aeryck