RAC Oracle Upgrade Question

I know this is probably answered in the Oracle RAC FAQ or something but i'm not a DBA just suppot them :)

When you need to upgrade RAC do you have to take the entire cluster offline ? Can do you do standby/dataguard between 2 RAC clusters ?

Can you have different versions of Oracle in a RAC cluster ?

Another questions can I have multiple databases ie multiple Oracle Homes on the clustered file system all running in a RAC configuration ?  say 5 versions of Oracle binaries all 5 being clustered against 5 systems each in their own RAC setup.

Thanks alot.

-- 
Unix Systems Engineer 
The City of New York 
Dept. of Information Technology
http://www.nyc.gov/doitt
rbrown[(@)]doitt.nyc.gov