My new Mac Pro has 6 "USB" busses, 12Mb/s, and two "USB High Speed" busses. 480 Mb/s. One of the "USB" busses shows Bluetooth on it, but none of the others seem to be in use. The external connectors belong to the two "High Speed" busses. Buss 0xfd has one of the front and two of the back connectors. Buss 0xfa has one of the front and one of the back connectors.
You're better off using an Mac-supported cards from Apple or others.
Apple's card are seriously outdated, both 5770 and 5870 are slow card in today's standard.
There "should" be a renew of Mac Pro some day, you can wait for that.
There are a cards like the Quadro K5000 that is Mac-compatible. But NOT out of the box. You need to install the driver first, and then install the card.
Also, support for AMD HD 7 series cards ARE NOT IN YET in 10.8.2. I had one of those in my old Hackintosh running 10.8.3, and it's still recognized as "AMD Radeon HD 7xxx series", so I will say wait for it.
Don't make any moves before AMD 7 series is out, maybe Apple or third-party will release cards for it.
Best Answer
No the first Mac Pro with a USB 3 port is the latest (6.1, late 2013)
From the specifications for your particular Mac Pro:
A quick Google search shows several PCI Express USB 3.0 cards, e.g.,