If you look at the big sticky thread and also use the search facility you will get plenty of answers. This has been extensively discussed over the past week or two.
I would advise looking at the MSI 680 4Gb Twin Frozr 2x 6-pin nVidia card or the ATI 7950 3Gb (PC version) which is also 2x 6-pin. You don't seem to mind the restrictions the PC non-EFI cards come with so those will both fit inside the Mac Pro's power specification.
If you are prepared to fit and use an external power supply you can go for the overclocked 680s and also the 7970s because they will need 1x 6-pin and 1x 8-pin for power.
