Graphics-Card and the Mac Pro
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Apple's card are seriously outdated, both 5770 and 5870 are slow card in today's standard.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.

Good luck!