Well Velvia is not a iso speed as such, it describes a film type (known for its vibrant colours and fine finish) but you are partly correct in saying that lower iso's will give you cleaner images (on the same camera). Not all cameras are the same though and some are much better at producing finer images at higher iso's than others are at lower iso settings.
In general you should select the lowest iso your camera offers that will still allow you to shoot and capture your intended shot using the settings you require. Its not always as simple as that and like everything compromises need to be made but that is the real basics of iso's/noise and digital cameras.
