You should probably update to QuickTime 6 for current games that use QuickTime. The games themselves may include QuickTime 6, or you can download it here
http://oldversion.com/program.php?n=quicktimeI've used QuickTime version 6.3 with both RHEM 2 and RHEM 3. I think Alida also worked with that version.
You can leave the System Restore as it is. The only changes you made were to QuickTime and Riddle of the Sphinx. Changing QuickTime to a newer version isn't usually a problem. It's changing to an earlier version that can be difficult.
I played Riddle of the Sphinx with QuickTime 3.02 at the time it came out. That was with Windows 98. QuickTime 4 had visual problems -- things in a drawer would be totally black in color instead of looking as they should. Anyway, if you want to try again with it, try it with QuickTime 3 instead of QuickTime 4. On a newer computer, you'll probably have to reduce audio and video acceleration.