Although the original post was back in April, I guess this is an ongoing question!
With the volatility in the Silver price, maybe a good way to look at it is like this:
Determine the loan amount you would have been comfortable with.
Determine the monthly repayments you would be making (if you took out the loan).
Buy Silver (or whatever) each month to the same value as the loan repayment you would have made.
(The "don't panic buy" is great advice I think. "Oh my god, silver is going to the moon, better take a loan!!!" might not be a good investment strategy!)
Everyone will have their personal degree of comfort for what they would do - I think I'd be reluctant to take out a loan, since there is only a reliance on capital gain and no cash flow. I guess if the market is clearly on a bull run AND you are sufficiently psychic to be sure it will continue, then it's a calculated risk.
Read "The Black Swan" by Nicholas Taleb (not an "easy" read but worthwhile) to make sure you aren't blind to any downsides.
http://en.wikipedia.org/wiki/The_Black_Swan_(Taleb_book) (Basic premise: You never know what unpredictable event might be coming down the track because ... well ... by their nature, they are unpredictable. Yes, he managed a book out of that, but it's still a good read for a great analysis of the lessons we can draw from our flawed reasoning and psychology that can eventuate)