If it's stamped, maybe it is sterling, but the older silverware is just as likely to be silver filled as it is 'sterling'. All 'sterling' means is that the contents are .925 % silver, and if you buy it, and it isn't sterling, in the US you can get your money back legally, one way or another.

I do this for a living, it is not all that easy to tell the difference, people in the business get fooled. So no, not a particualry good investment.
EDIT
I was thinking about this a little more. Generally it costs about 100 USD to refine a batch of precious metal, so if you have enough sterling whatever it was to send a few pounds to a refiner, and you bought the sterling at a price you are OK with, understanding that you might need to send it to a refiner for it to be investment grade, then you can still come out ahead.
The biggest issue with any precious metal is establishing a value for it, which is why people buy coins for investing in metals, because a coin is hard to counterfeit, which is why they have elaborate designs, and it has an established value. People don't make all of those elaborate coin designs because it's artistic, they are hard to reproduce. If you buy your silver flatware, it does turn out to be sterling silver, and not plate, and you send it to a refiner and get back .999 silver bars, and you are able to in turn sell your silver bars when you need to do so, then it would work.
The other issue is finding a refiner that doesn't skim considerably, and that also takes some trial and error.
Julie

I was listening to Mike the health ranger and he makes a good point that precious metal in currency form is the best thing to have as it is intrinsically valuable because its a precious metal, and it is also recognisable as currency. This is why he has a nice stash of gold coins.