this is my setup. I have PT 10 HD with a native card running on my 2.66 8 core nehalem (8 GB of ram) mac.
running symphony i/o
with one 8 channel a-d/d-a card (with additional 8 channels to an ai3 over adat).
the only problem I had was with the mac and setting the correct amount of cores to be used with pro tools (playback engine).
I've been running smooth as butter in both the mix and tracking world. had a session going yesterday with a bunch of drum edits and plug-ins (half mixed) and needed to record an accordion player. I set it to 64 on buffer size and was able to record her right into the session with no issues. I even had six channels being sent out to her for her cue on the hearback system. no latency detected on her end (at least audible enough to affect her performance).
this system seems really, really stable. and it's quite fast.