Generally, what almo says is true. But I have been hearing more and more that fast cards are making a difference with compact cameras. On the other hand, for most people, and most purposes, I don't think it will matter much. Personally, I'd buy the cheapest name-brand card I could find. A 256 or 512 would be the way to go. You get the best price/performance for your money at those capacities.

One place where the speed will matter is with a card reader. If you have a USB 2.0 card reader plugged into a US 2.0 port you will be able to transfer your files much faster. That's where I appreciate having a high-speed SD card.