The speed of the media isn't majorly important unless you're taking a lot of shots in a row. The camera itself should have an internal buffer which stores the pictures until it has time to write them out. Obviously if you take too many too quickly then you'll have to wait for it to save them.
Rob Galbraith's
CF Database page is useful. Even if you're not using CompactFlash cards, he brings up several points which are relevant for the rmedia types. Sometimes the manufacturers will switch controllers or flash chips part way through a product lifetime so what got tested as an amazing card by one person might turn out to be a mediocre card for another.