Apple shouldn't charge you until the order ships - however, for your order to be accepted, they'll almost certainly do a "pre-authorization" for the full amount. So if you don't have enough room on your card at the time you place your order, you may have problems.
Best to confirm with your CC provider - if you've been a good customer in the past, they may even increase your credit limit, after all, credit card companies ultimately make their money out of you spending as much as possible...
disclaimer: shambolic does not advocate or endorse irresponsible credit card spending, etc etc