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You won't lose any data after upgrading to Mountain Lion. It will keep all your data, but you should make a backup of your files before upgrading in case the upgrade doesn't finish correctly.
Also, check that your applications are compatible > http://www.roaringapps.com PowerPC applications aren't compatible in Mountain Lion and they won't run, so update them to the most recent version or remove the applications before upgrading
No. BUT... Back it up anyway.
Before attempting any Major Upgrade... get yourself an External Hard Drive... and create a Bootable Clone Backup of your current Hard Drive...
By far the easiest way to make such a Backup, is to use something like
That way, should anything untoward happen during the Upgrade,
you will NOT LOSE ANYTHING.
(Get an EHD that is at least equal to your current Drive...)
Also, Check here for compatibility of 3rd party Software you may be using...