64 simultaneous tracks (maximum) with the precious TDM Mix hardware. But you'll have to pay about 10 times as much as the Digi001 system costs.
With the TDM hardware, computer speed means almost nothing. A fast hard disk (or many of them) is a benefit. You know, when I'm running dozens of tracks and plug-ins with Pro Tools TDM, I have no activity shown on the CPU usage meter...And now, since PT 5.1 can use host processing (albeit in a little limited way), the computer's power is available for plug-in processing, which is often a real blessing.
But the price...Could I ever afford to buy TDM myself? Not in a lifetime, I guess...
Z