I installed CM6 rc3, including google apps, directly over Sapphire 1.0.0. I did not wipe/erase cache and have had no significant problems. Market automatically installed almost all my apps but I did have to manually uninstall and, then, re-install a couple of apps.
Running very smoothly. Doing a complete system/boot/cache/etc. deletion via ROM Manager. Hopefully, that will help.