I'm running card with sizes from 8-31gb and speeds from 300x to 600x and I think I also,have a couple 1000x Lexar 16gb cards, too. In my experience, they all perform fine in the camera since I'm not trying to shoot balls-to-the-walls 100 shot raw bursts.
The big thing is downloading to the computer. The faster cards, of course, take less time and that's the big bottleneck, IMO. However, downloading fast cards is limited by the both the card reader and your computer's bus speed. I am always shooting multiple cameras in the field so I always have several cards to download. My solution is multiple card readers, FireWire 800 and USB 2. Of the two, the USB is slower but for SD cards, that's the only option. I simply load up all the card reader slots and start the ingest going. With multiple card readers, this bottleneck isn't too much an issue for me. With USB 3 slowly taking over, this may then be an issue with slower cards eventually, but for now, save your money and simply buy and use slower cards.
In a couple of years, cards will be bigger, faster and cheaper and you'll probably replace them. Most likely card readers will be faster, too, and you'll probably have replaced both.