One of the most important reasons for packing a massive memory card is to enable you to shoot at your camera’s highest resolution. If you paid a premium price for a high megapixel cam, then get your money’s worth and shoot at high megapixels. And while you’re at it, shoot at your camera’s highest quality compression setting too.
Why not squeeze more images on your memory card by shooting a lower resolution and low quality compression settings? Because you never know when you’re going to capture the next great image of the 21st century. And if you take a beautiful picture at the low 640 x 480 resolution, that means you can only make a print about the size of a credit card, not exactly the right dimensions for hanging in the museum.
On the other hand, if you recorded the image at 2272 x 1704 (4 megapixels) or larger, then you can make a lovely 8- x 10-inch photo-quality print suitable for framing or even for gracing the cover of Time magazine. And just in case you were able to get as close to the action as you had liked, having those extra pixels enables you to crop your image and still have enough resolution to make a decent sized print.
The point is, if you have enough memory (and you know you should), then there’s no reason to shoot at lower resolution and risk missing the opportunity to show off your work in a big way.