The ATI Radeon HD 5770 available from the Apple store will work in your machine regardless to what it says on the Apple web page. Apple support confirmed this and I have just installed it my Mac Pro where it is working perfectly.
Apple's web site and support also state you need 10.6 Snow Leopard to use the card. I'm running it under 10.5.8 Leopard and it works though it may be using sub-optimal drives. As I brought an upgrade to 10.6 on Apple's recommendation I'm going to install that now.