actually IIRC the X1900 XT is an EBC card so should in *theory* work in a MP5,1 I know for a fact it works fine in a MP3,1 ("famous" youtube dude "its my natural colour" had one in his MP3,1 for a long time)
(of course it wont function properly post 10.7.5)
id actually like to get an X1900 XT or Xserve1,1/2,1 X1300 256MB PCIe BTO card and chuck it in my 5,1 then throw windows in EFI mode at it and watch the drivers utterly Sh*t themselfs trying to figure it all out
especially as I read the X1x00 ATI cards dont have a PC VBIOS but Just an EFI, (with the X1900s VBIOS being stored in the MP1,1/2,1s BootROM)