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.