The 2003-early 2005 G5 used the standard Airport Extreme card. From late 2005 - early 2006, it was an extended airport extreme card, same one found in the last 1.67ghz powerbooks. The new Mac Pro uses a very similar card too the Macbook/Macbook Pro, BUT sadly its an 802.11N card, which is the new format and the Macbook Pro card's arn't supported because of that.
So, make a long story short, there is no card on the market that will work in the Mac Pro at this time available to the public. Apple is shipping the correct card with CTO computers ONLY at this time. We are expecting the release of those cards any day now.