How did you solved your problem Captain Iglo?
What was the end result? I'm in the same boat mac pro early 2008 with dead 2600 XT card = no video. Options are:
a) buy an apple approved card for an arm and a leg
b) buy a used 2600 XT($90) and 4870($100) card to flash
c) pay over $250 for a used 512MB or over $350 for 1GB flashed 4870 card on ebay