Call your credit card company. Tell them what's happened & ask them to cancel the transaction. Most cards will have a department that deals with scams/fraud. They'll advise you about whether you should cancel the card.

Have a read of the 2 threads below. The first has basic info about being scammed (it's very long, just read as much as you can take!!). The second has info about how some people have been able to get a refund from scammers. If your CC company can cancel the transaction & deal with things, you won't need to tackle it yourself...but just in case

http://www.gnutellaforums.com/showthread.php?t=28751

http://www.gnutellaforums.com/showthread.php?t=44499