CF Cards

1) It must be a card type compatible with your camera. If you're using an older camera, be sure to read the manual (or manufacturer's website) first. Look for the part of the manual describing compatible CF card types. For instance, some older cameras cannot support CF cards with a capacity over 2 GB.

2) Write speed: This is the speed at which data is written to the card once a picture is taken. You'll obviously want a fast write speed, but another factor thrown in will include your camera's read & write speed. In other words, a fast CF card and a slow camera won't exactly make for an optimal end result (although, it won't hurt your camera or the card).

3) Storage space: A self-explanatory factor. Just like PC hard drives, you'll want the largest size card for the work you'll be doing. If you only take pictures at family events and holidays, obviously - you won't need overkill in terms of storage space. However, if you're taking high resolution pictures for your portfolio with a modern SLR camera, then you'll want the top-of-the-line card only.

Lesser CF Cards are now becoming outdated, as 1GB cards appear to be the new "low standard." Here is a chart outlining the minimum requirements based on your camera type:

4 megapixel (or lesser) cameras
-CF card speed: not applicable for this camera type
-CF card storage space: at least 128 MB

5 megapixel cameras
-CF card speed: 32x-50x
-CF card storage space: at least 256 MB

6 megapixel cameras
-CF card speed: 40x-60x
-CF card storage space: at least 256 MB

7 megapixel cameras
-CF card speed: 50x-80x
-CF card storage space: at least 512 MB

8 megapixel cameras
-CF card speed: The fastest possible card
-CF card storage space: at least 512 MB

9 megapixel cameras
-CF card speed: The fastest possible card
-CF card storage space: at least 1 GB

10 megapixel (or higher) cameras
-CF card speed: The fastest possible card
-CF card storage space: at least 4 GB

Source: Dubbin, R., Grunin, L. Picking the Right Card for Your Camera (2007). CNET Networks, Inc.