Throne Room + Mine

Throne room with a mine instantly allows you convert a copper into a gold and it goes right INTO your hand so you can use it right away.  You can quickly convert all your copper into gold and you start buying provinces!

Mine

Description: Trash a Treasure card from your hand.  Gain a Treasure card costing up to 3 coins more; put it into your hand.
Type: Action
Cost: 5
Set: Dominion

This a great card by itself as it allows you to upgrade your copper -> silver -> gold.  And don't forget that it goes into your hand RIGHT AWAY.  When this card is in play, there is no longer a need to trash those coppers, but rather you can upgrade them to silver.  Get a copy of the mine card early on in the game, and you can be sure to quickly turn your coppers into gold soon.  There probably isn't much of a need to get more than 1 or 2 of these cards as it becomes useless near the end of game as you will have already upgraded most of your cards.