ATI Radeon HD 5770 in MacPro 1,1

I've searched all over these forums, there was some talk about the ATI Radeon HD 5770 working in a 1,1 MacPro, but I never really found a definitive answer. In the apple catalog someone posted this:

"It should be fine ATI cards unlike nvidia are built to be compatible with both EFI 32 and EFI 64 interfaces and i know the 4870 is fully compatible even with the first generation intel mac pro some people were saying that the 5770 takes up 2 pci express slots but that is just false i havent had the chance to try it myself since my 4870 is enough for me but i am sure it should work just fine"

Any final word on this? I'm eager to upgrade.