The major advantage of using a credit card for purchases over the internet is not the credit limit, but rather the insurance that you automatically get with it. If you go to your bank and say "this guy ripped me off, I want my money back" and can prove it, then you automatically get your money back and then the bank has to deal with it. When you use a debit card and you don't get what you want, you have to mess around with it yourself and it can spiral out for ages (if anyone remembers PA's "slammer" PS2 deal, you'll know what I mean).