To be perfectly frank, if you're not destroying great bundles of credit cards (or any other money-replacing cards), all you actually need is a good pair of scissors or tinsnips. Cut once along the magnetic stripe on the back and once somewhere else and the thing will be destroyed beyond hope. But if you must shred any great quantity, then any medium- or heavy-duty shredder which is capable of handling CDs will do the job for you.
Why bother? Well, it isn't likely to happen, but it is possible for evil technological wizards to read the magnetic stripe, decipher what's there, and update it to match the bank's current information, thereby allowing access to your account. If you do want to keep them, at least disable your cards by scratching the stripe.