Apple Mini DisplayPort adapters: Frequently asked questions (FAQ)
http://support.apple.com/kb/HT3382
Guide to Apple graphic display adapters needed
https://support.apple.com/kb/HT3235
Atlona (Dual Link) DVI to Mini DisplayPort Converter for Apple 27-inch LED and 27-inch iMac
http://www.atlona.com/Atlona-Dual-Link-DVI-to-Mini-DisplayPort-Converter-USB-Pow ered.html
CS6 with that much memory you are forcing it to 64-bit mode and all your drivers and plug-ins should be as well.
10.8.3 supports more CUDA capable graphic cards.
The reason for 10.6.8 is to stay with Rosetta.
I would dual boot and put 10.8.3 on its own hard drive - call it testing - and never update over the old, Setup Assistant will bring those apps etc over. You can clone if you want to insure you have more bootable backups too.
Kind of hard not to install anything even if all you do is the software updates offered which can affect other things as well, which security updates have a habit of and breaking something else in the process.
I think everyone should be familiar with all the support articles branching off main support page
http://www.apple.com/support/macpro
https://support.apple.com/kb/index?page=articles