1) Dispute charge with your credit card issuer (as others have recommended)
2) Check out the consumerist blog (consumerist.com) and use their guidelines to get consumer satisfaction.
Don't let them BS you - put the beef out in public and you're more likely to get results. Dell and other large companies don't care about you, an individual consumer - make it public and affect thousands of buying decisions and you'll likely fare better.
Note: If they resolve this to your satisfaction, also post/email/whatever a follow-up showing that they made good on a bad situation. If they do not, of course you should let everyone know that as well.