According to this Wiki ( »
en.wikipedia.org/wiki/Mac_Pro ), the 2009 Mac Pro came with either a GeForce GT120 or an ATI 4870 video card- the ATI 5770 was not available (from Apple) until the 2010 model.
Since (according to the support article) all 2010 MP's can output Mini DisplayPort audio, I assume that all (Mac-version) 5770's would work. Cannot guess about reflashed Windows-version cards.