I have done the 64 bit hack and it works fine. Perfectly stable.
You are using an alternate boot loader for your computer that would normally be used for Hackintoshes. You are essentially turning your genuine Mac into a Hackintosh.
There is nothing about the 2006/7 Mac Pro that can’t handle 64 bit operation except That Apple uses an EFI chip to limit Mac OSX to Macs…the chip used in the early Mac Pros can only hold 32 bit code. The rest of the computer is not a whole lot different than later Mac Pros…Xeon processors , fast ram..etc.
Just because it worked for me doesn’t mean it will work for you. There are a lot of variables…and there are only a few hundred people on the planet that have even tried it. Most of them like me and the guy who wrote the blog I link to, learned how to do it via the netkas.org website. I did it as a project to see if I could. The machine I hacked is not my primary source of income.
If you have DX hardware and absolutely need a stable system to make money on, I would stick with Avid 5.x .
Blackmagic quirks….sluggishness in trimming in the trim window, , video unstable when trimming with caps lock audio scrubbing on, no universal mastering. Perfectly usable, but not as good as DX hardware.