Just because your digital camera uses SD Cards to store photos doesn't mean that it will work properly with just any SD Card. Older cameras can't recognize new Secure Digital High Capacity (SDHC) media, which can hold 4GB to 32GB of data and cost as little as $50.
Older SD Cards have typically maxed out at a storage capacity of 2GB. Some 4GB SD Cards appeared prior to the new high-capacity format's release, but using these cards isn't recommended, due to compatibility issues. Unfortunately, most digital cameras sold before summer 2006 are unlikely to be SDHC-compliant (though a couple of models can be made compatible with a firmware upgrade).
If a camera (or other device) does not state support for the SDHC format (look for the SDHC logo on the box), you won't be able to use the higher-capacity cards with it.