[This other question](https://photo.stackexchange.com/questions/325/how-can-i-know-what-speed-card-to-get-for-my-camera) covers how to figure out what speed you really need. But to answer the _other_ part of what you are asking: **No, card speed does not affect image quality in any way.** The image files are digital, and it's not like analog cassette tapes where the composition of the media can make a difference. The only case image quality could be affected is when the slower speed forces you to choose a lower-quality setting in order to make smaller files. But that's a choice you'd make intentionally.