The Card speed is important and will become even more important in the near future as we move into higher resolutions. I have noticed that some Cameras like the Pentax Optio 43WR needs a faster SD card in order to give you a smooth Video. This will be even more prevelant as camera make their movie mode VGA or higher. Some of the best cards I have seen on the market would be the Panasonic SD cards. They may not say 30x 45x or 60x which doesn't really give any accurate indication of speed unless you look at like the CPU Speed differences. Panasonic I think has the fastest SD card on the market with their new SD PRO the operates at 20MB/Sec. If you looked for a Lexar card that would do that speed I think they would need to label it 120x speed.
I hope this helps