As Oxide said, all you need to do is have the card on your account and as long as there is a balance on the card it will go through just like a purchase in a shop.
I don't know, it also dependes what type of card you have. Because I have a Maestro debit card which is were I put all my savings (and use sometimes to buy things when I go out) but I can't link it to PayPal.
For me, I've to go to the bank and do a transfer.
When you pay for an online item, Paypal takes whatever money is in your balance to fund your payment. If there isn't any or there is not enough, it takes the remaining portion from your bank account or credit card, which ever one you have linked to it. So, as long as you have the card hooked to your paypal account, you're good. Hope this helps