Either stick with leopard (10.5.8) - as everything seems to work ok
or
Snow Leopard 10.6.0 / 10.6.1 [CORRECTION] or 10.6.2 if you are not worried about video input by cards other than Active Silicon
See post about 10.6.2 QT video input issues:
http://chaldee1.gotadsl.co.uk/~richa...ead.php?t=1781
Down-grading is only an option with a non 2010 MacPro - any current versions available from apple as of Aug 2010 cannot be downgraded to previous software version.
With your Mac spec - there is no issue with 64bit - as its only 32bit...
Your issues with HD (or other) playback is not strictly to do with quicktime being broken in 10.6.4 - its more to do with the ATI graphics drivers being broken
If you have the option - try installing an Nvidia GT120 card and do a direct comparison - dont change anything else - and see what results you get.
Generally Snow Leopard gives better performance gains compared to Leopard - except 10.6.4 where the ATI graphics drivers appear to be broken...
For 2010 MacPros there are good workarounds the current issues - and performance does not suffer too much if you follow the suggestions.
(boot in 32bit mode, use an nvidia graphics card, Active Silicon Video input)
See other posts for details:
http://chaldee1.gotadsl.co.uk/~richa...ead.php?t=1871
and
http://chaldee1.gotadsl.co.uk/~richa...ead.php?t=1875
and
http://chaldee1.gotadsl.co.uk/~richa...ead.php?t=1781
Simon