Just thinking about a major upgrade of my system, changing my HD2 with a single HDX with PT11, my 2006 mac pro with a newer model like 2010/2012 series.
I'm wondering if this system will work with an old Apogee AD/DA, like the rosetta 800/96Kz with the X-HD card.
Yes it works. I'm running PT11 HDX with X-HD cards into Apogee Ad16x/DA16x, but it should work the same with the Rosetta. You just need to visit the Apogee website and update the firmware for your X-HD cards (I think it's Version 3 now that you want). That allows the X-HD cards to emulate the newer Avid HD I/O instead of the older Blue 192's.
Oh and you need to make sure that you get the right cable to connect your HDX card to the Apogee, because the HDX card uses a newer small Digilink cable but the X-HD card uses the older big one. They have converter cables, just need to make sure you get the right combination of male/female jacks.