This other questionThis other question 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.