Q: new graphic card for Mac Pro 2008 3.1

GTX 680 (PC, flashed by MacVidCards or yourself or EVGA Mac Edition) or any of the cards since, up to and including GTX 980 (requires Yosemite already installed).

Or AMD 7xxx series R9 280 and above.

If you want to go with an ATI card, there is a 3GB option.

Other World Computing (long time Mac 3rd party reseller) is offering the ATI Sapphire HD7950 Mac Edition.

http://eshop.macsales.com/item/Sapphire/100352MAC/

No need to flash this graphics card and though it's a little pricey (at just under $480), OWC will buy back your original Mac Pro card to lessen the blow to your wallet. I replaced the ATI Radeon HD 5870 that came with my 2010 MacPro and OWC bought back the old card for $200. Good performance boost in both Mavericks & Windows 7 and it runs quiet.

Check with OWC on compatibility

mayaman - just realized that you have a 2008 and I have a 2010 Mac Pro

Which means you don't get as much return in performance in a 2008 as 2010/2012.

And there are more affordable and some go PC route $200

http://www.amazon.com/Sapphire-DisplayPort-PCI-Express-Graphics-21196-00-20G/dp/ B00BXVFM3K/ref=sr_1_2?s=pc&ie=UTF8&qid=1…

Radeon HD 7950 for Mac Pro versus other GPUs
Radeon HD 5870 outperforming all GPUs including the 7950. The Radeon HD 7950 was beat by ... 3. The Radeon HD 7950 is not a revolutionary ... NOTE: The Radeon HD 7950 is compatible with all
barefeats.com/gpu7950b.html

benchmarks for the Radeon HD 7950 for Mac Pro
Let's get right to the test results. GRAPH LEGEND Radeon 7950 = Sapphire AMD Radeon HD 7950 GPU (3G VRAM) ... the Radeon HD 7950 is a significant ... The Radeon HD 7950 is clear winner
barefeats.com/gpu7950.html

Radeon HD 7950 for Mac Pro versus even faster GPUs
Twitter SHOOTOUT WITH THE 'SHARKS': Sapphire Radeon HD 7950 for Mac ... Sapphire Radeon HD 7950 Mac Edition does ... 3G GDDR5)* Radeon 7950 = Sapphire AMD Radeon HD 7950 GPU (3G GDDR5
barefeats.com/gpu7950c.html

Fast GPUs on older Mac Pro versus newer Mac Pro
GRAPH LEGEND 2010 + 7950 = Sapphire AMD Radeon HD 7950 in 2010 ... 6-core 2009 + 7950 = Sapphire AMD Radeon HD 7950 in 2009 Mac ... core 2008 + 7950 = Sapphire AMD Radeon HD 7950 in 2008
barefeats.com/gpu680v4.html