Just to further add to this thread...
I've been considering this upgrade for well over a year now, and finally took the plunge.
It all works great - it's as if I hadn't changed a thing (albeit now I have a 27" ACD!)
I'm not using it so much for graphics, but I needed a card that would work with the mini display port, so I can't really comment on advanced graphics performance. But it has worked for the past two weeks without issue!
I will add these two bits of advice:
1) the day before buying the card, I called Apple tech support, and they weren't very helpful. Firstly, he said the card physically wouldn't fit in the MacPro 1,1 (I bought mine in 2007, so I'm assuming it is the 2006 model). Of course, I came to find out it fit no problem in slot 1.
2) He said given the card is PCIe 2, and the MacPro 1,1 is PCIe 1, it definitely wouldn't work here either. However, I did a bit of research (simple google search, barely research), and found out that PCIe is forward and backward-compatiable. ie, PCIe2 card would work on a PCIe1 motherboard. No issue there either.
So I risked it and it worked! Word of warning: whilst the display is probably returnable with full refund within two weeks of purchase, the grahics card, once out of its static bag, is non-returnable/non-refundable. You can probably sell it on eBay for the same sort of price (or many people in this thread even) if it doesn't work for you.
Good luck, and thanks to everyone who has posted before me giving great advice!