Apart from size and capacity of memory cards, you also need to check the 'class', which determines how fast you can write to the card. This is important for devices that can record HD video. Check the product manual to see the card spec you need.....
Except "class" is a very badly defined test. It's about burst speed rather than continuous - and if you're recording HD video, you're wanting continuous speed, not burst.
Here's a few speed tests for "Class 6" SDHC cards:
Transcend Class 6... 4GB...* 8.3MB/s
Transcend Class 6... 8GB...* 7.6MB/s
ATP 150X Class 6... 16GB... 7.5MB/s
Transcend Class 6... 16GB...* 7.2MB/s
Kingston Class 6... 8GB...* 4.1MB/s
PNY Class 6... 4GB...* 5.1MB/s
Kingston Class 6... 4GB...* 2MB/s
If you're wanting to get a card for serious photography etc, stick with Sandisk Extreme or Lexar Pro - don't skimp on it.