What camera? A point and shoot at 12mp will have a higher default compression than a DSLR at 12mp.
Using a point and shoot, you will get a minimum of 1,000 shots on a 4gb card. With a DSLR, you may only get 800 at Fine JPEG setting.
You should never use your camera at a reduced resolution, the pics will look like crap and just be a waste of time and effort. Get everything your camera is capable of capturing! Memory cards are cheap, a 4gb can easily be found for less than $20. You should have a couple of them anyway.