Just to add,
I had the same setup as the OP. A quad core 2.66 1st Gen Mac Pro with a 7300, a 30" ACD and a 23" ACD. The 7300 drove the 30 inch at full res and the 23" at 1920. It was a little hiccupy doing anything graphic however.
I ordered the XFX 4870 from Newegg for 150.00 bucks. It has twice the DDR memory (1 gig to 512 for the Apple card), is 200.00 cheaper and has 2 DVI ports instead of a mini display (and another 100.00 for the mini display to dvi adapter)......
I ordered the power cables from shop.ati.com.
The flash process was relatively easy, there is a great step by step walk through out there (I just deleted the bookmark I had since I'm done with it.).
The only thing I did differently was I didn't install XP in bootcamp for the lone sake of backing up the rom on the card before I flashed it. I figured if it blew up, someone out there has the rom anyway.
I made the ultimate boot CD in VMWare, added the necessary files for flashing the card, plugged the card into slot 1 and the 7300 into slot 4.
I connected the 23" ACD to the 7300 and left the 30" unplugged, booted into DOS, flashed the card, powered down, plugged the 30" into the 4870 (and left the 23" in the 7300) and rebooted. No problems at all, the card is quiet as can be.
I didn't fully push it yet, but I ran glxgears which gave me a reading of about 4400 FPS on the 7300 when the window was on the 23" and 11000 FPS on the 4870 when it was on the 30".
Also, as an FYI from what I understand the XFX cards are the only PC 4870s that both DVI ports work after flashing. You get one single link DVI, one Dual link DVI and the card doesn't work with VGA adapters for some reason which is no big deal to me as I still have the 7300 sitting in there if need be.