netkas can i use HD2600XT instead 8800gt?step by step about software side - http://netkas.org/?p=106
well, you can try to install drivers with 8800 gt installed, then shut down macpro, insert 275gtx and try to boot it ( u will not see anything until os booted)
if it doesnt work - swap 275 to 8800gt and it should boot.