If you dont like minidisplay port or dont want to pay 3 times more for a card from apple, here is directions for how to get 4870 card working in macpro.
1) go to here and download 4870.ROM.zip file, and unpack it
2) backup rom of 4870 card, you can use latest gpu-z utiliy for it
3) flash you 4870 card using dos usb stick and atiflash
atiflash.exe -p -fs -fp 0 4870.rom
I’m not going to explain how to do it, all info is available over net, use google.
4) install drivers pkg for 4800 from #radeonhd channel on irc.osx86.hu (first link in topic)
after installing pkg remove /System/Library/Extensions/Natit.kext
5) reboot, should work just fine

both dvi ports works.
it was tested with standart card (2dvi and 1 svideo ports), nobody tested with nonstandart cards (with vga/hdmi/DP ports)
known problems : vga (with dvi2vga) doesnt work.
WARNING: I’m not responsable for any damage you can do to your system, Do it all at your own risk.
P.S. once you update to 10.5.7+ u can forget forever about step 4