The answer is my favorite answer -- "well that depends".

The answer used to be relatively simple. Most cameras used the same type and degree of compression and supported very few card sizes (since few were available). Today, the answer to the question is extremely complex. The best way to answer this question is to consult your cameras manual. There is usually a table that you can use as a guide. The major considerations for storage are:

Storage Card Size
This is a simple one, the storage capacity is usually written right on the card. The available storage is typically 8, 16, 32, 64, 128, 256, 512 MB and 1, 2 and 4 GB. These are not the only sizes available and are available in several formats (compact flash, secure digital, etc). Again, please consult your manual or the manufacturers web site for more information on what sizes can be used with your camera.

Image size setting
Most of todays cameras allow you to select the image size that you are taking. This setting is one of the most important in determining how much you can store. There are typically 3 to 6 sizes that your camera is prepared to make. These sizes are typically quoted as "Large, Medium and Small", where "Large" refers to your cameras megapixel rating and "Small" typically refers to 640x480 which is the typical 'web/email' image size.

Compression Mode The final component in determining how much you can store is your cameras compression mode. With some cameras, you can't do anything to change this, but in many newer camera models you can change how much each image will be compressed. For example, most cameras have a "RAW" image setting, which is no compression at all as well as JPEG compressed images. For a lot of older digital cameras, these are your 2 only choices. In newer cameras, there's several different compression levels which really cloud the ability to answer this question.

There are 2 things that you'll need to keep in mind:
1 - The exact number of images that you can take actually depends on the type of pictures you take. The more various colors that you take, the less that your images will be compressed -- no 2 images will compress to become the same size. Your camera may say that you can take 1 more picture on your card, and you may be able to squeeze another 2 to 3 pictures onto the card, OR you may not able to take any.

2 - One thing that you need to keep in mind is that once you take a picture, the amount of storage that your image takes up will not change if you change the settings on your camera. The camera settings only change these amounts for images taken with the new settings. The only way for you to take up less space on your storage card after you've taken pictures is to delete images.

To provide you with an example of how complex this question can be, the table below illustrates the # of recordable images as they are portrayed in the Canon PowerShot Pro 1 camera recently released by Canon.

NUMBER OF RECORDABLE IMAGES

Menu Selection File Size (KB) Resolution (pixels) 32MB 64MB 256MB 512MB 1GB Large/SuperFine 3,436 3,264 x 2,448 8 17 72 144 300 Large/Fine 2,060 3,264 x 2,448 14 29 120 240 496 Large/Normal 980 3,264 x 2,448 30 62 252 503 1,024 Medium 1/SuperFine 2,002 2,272 x 1,704 14 30 123 247 512 Medium 1/Fine 1,116 2,272 x 1,704 27 54 222 442 910 Medium 1/Normal 556 2,272 x 1,704 54 110 443 879 1,726 Medium 2/SuperFine 1,002 1,600 x 1,200 30 61 246 491 993 Medium 2/Fine 558 1,600 x 1,200 54 109 440 879 1,726 Medium 2/Normal 278 1,600 x 1,200 108 217 868 1,734 3,283 Medium 3/SuperFine 570 1,024 x 768 53 107 431 855 1,726 Medium 3/Fine 320 1,024 x 768 94 189 762 1,522 2,982 Medium 3/Normal 170 1,024 x 768 174 349 1,390 2,741 4,687 Small/SuperFine 249 640 x 480 120 241 962 1,891 3,845 Small/Fine 150 640 x 480 196 393 1,563 3,112 5,468 Small/Normal 84 640 x 480 337 676 2,720 5,203 8,202 RAW 9,020 3264 x 2448 2 6 26 54 115