For a visual card change, you would be hard-pressed to come up with something more impressive than "Believe". The spectator chooses a card and signs it, the card is lost and another pulled out. After tearing a corner off and placing it on the table, the magician suddenly changes the corner of the indifferent card into the corner of the chosen card--without ever apparently touching it! The torn card is turned over and it is seen to be the signed card...even though the spectator saw it as a different card just before it was laid down.
I'm not usually very high on one-trick DVD's that cost $20 or more, but I have to make an exception here. The change is so visual that there's a good chance you'll hear your spectators actually gasp when it occurs.
Joel Paschall does a great job of teaching this effect and the moves involved. Even the clean up is relatively easy and of course it's ready to go at a moment's notice. Understand something: this will take practice. There are 2 key moves you'll have to have down pat before considering this, and Joel's one-handed double-lift (yeah, you read that right) alone will take some time. Once you have it down though, I guarantee it'll be the only double-lift you use.
This will require a table to perform properly, so this isn't really a stree magic effect. If you're into table-hopping though, this is something you seriously need to consider. Actually, it's something every serious card worker should consider.