Dual Radeon 5770s. I'm actually using two PC versions of the cards. The tails are too long to fit in the Mac Pro, so you will have to use a hacksaw to shorten them. I know you just LOLed when you read that, but it works and it saves you a few hundred dollars instead of buying the Mac versions. Both cards work great and I got them for around $175 together on eBay.
If you don't want to bring out the hacksaw, then I still would suggest getting two of the Mac versions. You will just pay a few hundred more for the set.
I just stuck PC GTX680's in our old MacPro's - all it needed was an extra PCIe power cable via eBay.
The only downside is that you don't get any boot screen - just the OS-X login windows onwards. This could be a problem but after doing it a year ago I've never once had the machine completely fall over and wish I could see the boot 'grey' screen.