For Pro Tools HD 8.x and below, an Avid Pro Tools|HD (TDM) hardware system is required to use Pro Tools HD software:
- A Pro Tools|HD Core card
- A qualified Pro Tools|HD audio interface
You may be able to use a third party audio interface that is recognized as an Avid HD audio interface. But you would still need the Pro Tools|HD Core card regardless, in addition to the audio interface.
Also, see the grid for Mac OS X versions for Pro Tools HD 7.4 on this page:
Pro Tools HD for Mac OS X Compatibility