ITunes gift card

According to Apple's page, it uses the following order for song purchases:

• Song credits.
• Gift certificate, prepaid card, or allowance credits.
• Credit cards.

Thus, it would appear that you cannot charge a credit card without using up your credits or gift certificates first.

However, similar to what @Nick said, you could just open up another account, add the credit card to it, and then gift the items you want to your original account.

Update: Based on my own tests of self-gifting, these are my findings:

• Gifts will deduct money from the credit card on your account; they will not use up song credits nor gift card credit. This means that you do not need to open another account, just add it as a credit card and gift it to yourself.
• You cannot send a gift to the same email address as your iTunes account. This can easily be circumvented by sending it to one of your other email accounts.
• The easiest method of gifting to yourself is to select the print option. It will show you a code which you then use to redeem the song. It will allow you to redeem the code on the same account.

I think this will work...

Open iTunes

Just below the volume bar is your Apple ID.

Click on that and select Redeem. It will prompt for your Gift Card info..

Here's a little pic: