The older stuff may FUNCTION in our 64 bit world, but as to how well....that's somewhat of a guess. Certainly, you can run TDM cards on a 64 bit OS, but the hardware gains no benefit. Pro Tools 8.1, 9 and 10(all versions) are compatible in that they all work, but when I moved to Pro Tools 11(a fully 64 bit program), that made a big change in performance. But, TDM/HD cards won't work with PT 11(not sure if that's because they really can't work for some reason, or if Avid just decided they were not going to spend any R&D funds to make it work)
Given how many great interface options we have today, its worth looking around at other options. Plus, its still possible to use 192's with current HDN and HDX cards, and some other boxes like Waves StudioGrid(or whatever their box is called
). I have a 192 happily working along side of a pair of HD IO boxes on my HD/Native rig