You need to have Lion 10.7.2 or above to have iCloud...your iMac will run that. To check on use of Mountain Lion go to http://www.apple.com/osx/specs and see if your iMac is on the list...MAS should not have let you download if it was not compatible.
All you need is to have 10.6.8 to install Mountain Lion on a compatible machine.
Note your PPC based software that was running under Rosetta will no longer run in 10.7 or 10.8
check here before you burn.
Also you should Carbon Copy Cloner your 10.6 boot drive to a blank external Disk Utility GUID OS X extended J formatted drive BEFORE you upgrade to 10.8, because it's really hard to go back to 10.6 manually.
With a clone you can hold option keyy down, boot off the clone, erase the entire drive and reverse clone 10.6 back on just like it was.