Many modern manual cameras can cope with very high ISO. In fact, some night sky photographers would never use anything less than ISO 3200. However, older crop-sensor cameras almost always produce cleaner shots when used at ISO 800 or below. Just remember that what the image looks like on your LCD screen isn't necessarily a good representation of what it will look like on your computer. Settings like ISO 6400 may look great on your camera’s LCD screen, but it’s risky unless you have a very new camera.