A 3-megapixel camera should have at least a 256 MB card; a 4-megapixel camera needs at a minimum a 512 MB card. Any camera with 6 or more megapixels should have a 1 GB card. This will eliminate the possibility of ever missing a shot because your memory card is full, or even worse, having to choose which photo to delete because you have found one more that you just have to take!
Being able to shoot at your camera's highest resolution is another important reason to have a massive memory card. Spending money on a 6-megapixel digicam should also include shooting with all 6 megapixels!
Another thing to remember is to shoot at the camera's highest compression setting, too. Squeezing more images onto your memory card by shooting at a lower resolution and with lower quality compression settings will only cause regret later. You never know when you will shoot the next great photo. (Do you think the photographer left home that morning knowing he would photograph the nurse kissing the soldier? That photo turned out to be the most well-recognized photo taken at the end of the war!)
Don't chance missing a great shot with poor quality. Another thing to remember is that the lower the resolution of your photos, the smaller the print will have to be to remain clear. No posters can be made with a 640 x 480 resolution. Not exactly what you will want to display at your first gallery showing!