Should I buy Gold or Hoard?
One common mistake people make is picking up a Hoard right away, and then buying a Duchy (or even Estate) to get the extra Gold. When you do that, you have added 3 cards to your deck, for a total of $5 money. That's less money-per-card than just a Silver gives you! Each time you buy a Duchy and get a Gold, you're adding 2 cards for $3 money... again, worse than Silver.

But Hoard certainly has it's place. If there are other useful Victory Cards, like Island or Tunnel or Nobles, then Hoard becomes great. Also, if you think that it's likely that a Gold would give you $9 to spend on the turn you see it, then Hoard will probably be better, because then you'll have $8 to spend, which is still enough for a Province.

To figure out how likely it is that you'll be able to get a Province, it really depends on the rest of your deck. If you have lots of other money, or you have a good engine that can draw a lot of cards, then it's a pretty safe bet usually that buying Hoard over Gold won't cost you that Province. A general rule of thumb that I personally follow on most boards is to buy a Gold first, and then Hoards after that.

Here's a good article on Hoard strategy: http://dominionstrategy.com/2010/12/02/prosperity-hoard/