Is it best to unload Vegas 8.0 and then load Vegas 9.0 or just load 9.0 ontop of 8.0???

Keep your previous version there until you're ABSOLUTELY positive that V9 has no bugs that will mess you up.
If you plan to work on older projects, save them under a different name so that, if you need to, you can still open them up with V8.
I've still got V7, V8 and V9 on my computer and they get along quite nicely :-)

Just as Mike says you can run several versions at the same time, no problems, I have three versions installed with no issues.

To further explain, 9 will NOT load "on top of" 8. Instead, it will load BESIDE 8.