Scan at the highest resolution you can cope with, and then scale it down. JPG is best avoided for things like sheet music; it's a "lossy" format and will degrade in quality each time you re-save it. Gif is more reliable, though it makes bigger files than seriously compressed jpg. If you have ABC2Win, get it to make the staff notation for you: don't print and scan. It doesn't cost much to register the software and get full functionality.