I totally agree with maflynn. I did an upgrade to SL and it went quite smoothly with a well maintained Leopard install, but it only takes a little extra time to clean install & have peace of mind, in which case I would also suggest that you do not re-import your entire ~/Library backup. Rather it would go even smoother if you export your address book, bookmarks, email account settings, etc., back them up onto your backup drive, then after the clean install, re-import aforementioned major settings only. Install your third party apps by getting the latest versions from the software developer. I managed to only need 3rd party stuff that is compiled for SL & 64-bit.