If you have a 4 or 5 MP camera and if you are shooting in fine JPG format, then you should have memory cards that store at least 256MB, while you should probably have at least 128M for a 2MP camera.  In each of these cases, you could store about 120 images if your camera setting was set to the "Finest" JPG (which is what most people should use).  Many people should have double that amount of memory (or more).   More memory is better, particularly if you are traveling out in the middle of nowhere.  If you are shooting in TIFF or RAW format, you need even more memory.
 

Storage Mediums:

• The most common Memory Cards are Compact Flash, Smart Media, and Sony Memory Stick.  Compact Flash is used by a few more cameras than the other two, it is slight larger than the other two, it comes in larger capacity (4GB), and its price is slightly less.  In general, these three storage mediums are similar.  For a 256MB card, the cost is in the $50 to $80 range.  (Note that 4 years ago, the cost for 32MB was $50, so expect a decreas in price in the future.)   Some other memory cards that are also used are the XD picture card, the SD secure digital card, and the Multimedia card.  Most cameras will use only one of these types of cards.  The new Sony high end camera uses either Compact Flash (Type II) or the Sony Memory Stick. The performance of all of these cards are similar.
• Compact Flash makes a Type II card that can operate much faster than the basic Compact Flash card.  For a 256MB card, the CF Type II card costs about an extra $10.  I recommend spending the extra 10 bucks for the faster card, if your camera can use it.  (Note that many of the lower end cameras that use Compact Flash can not use the CF Type II cards.)  Sony Memory Stick also has a card that is about the same speed as the CF Type II card, but the supped up 256MB Sony card costs an extra $50.
• IBM makes the MicroDrive, which can store several GB's and it is costly.  It works on cameras that use Compact Flash Type II cards.  Even though these can store a ton of images, there are some disadvantages to the MicroDrive.
• Sony made cameras that used a 3.5" Floppy Diskette.  This is cheap, no software is required for download, there is a long delay between shots, and they can store only 8 JPG images at 1MP resolution and 3 JPG images at 2MP resolution.  You can not store a 5MP image on a diskette.
• Sony uses Mini CD's in some models.  The memory is cheap for the number MB's.  However, there is a long delay between shots because it basically needs to burn a CD.   This is useful in a setting where the camera is used by a wide variety of different people.
• Some cheap cameras do not have removable memory cards.  These are "cheap cameras."