No, you can't. The Phase 28 is (so far as I can tell) a PCI card, while the Mac Pro only has PCIe slots, which are not backwards compatible.
The last Mac that shipped with a PCI slot was one of the G5 towers that preceded the Mac Pro; if Terratec has a driver for the card that's compatible with a G5 Mac, then you could buy one of them and use it.
There is a theoretical solution in the external PCI chassis that Magma sells, which can essentially convert an internal PCIe slot (via a card) into an external PCI slot:
http://www.magma.com/1slot.asp
...but that would rely on both the external chassis working properly, and Terratec having a driver compatible with MacOSX 10.6, which seems unlikely given that it's exceedingly difficult to use that particular card with any Mac capable of running 10.6 (which is to say an Intel Mac, none of which have ever shipped with PCI slots).
Regardless, given that the above costs about $1100, and you're talking about a computer costing upwards of $2000, you'd do a whole lot better to just buy a new audio capture/output device that's guaranteed compatible. The Terratec DMX 6Fire USB, for example, is Mac compatible, and seems to have a similar 4 out/6 in @24/192 to your card for about $300, and there are plenty of other higher-end, Firewire-based, low-latency audio products available. Heck, you could even buy an iMac instead of a Mac Pro, if it will otherwise fulfill your needs.
