This may have been confirmed since I did this upgrade back in January (I couldn't find the info when I initially researched the upgrade). But here is a video confirming:
http://www.youtube.com/watch?v=rgbnE_NVuVY
- The Radeon 5770 does work in a Mac Pro 1,1
- Dual Radeon 5770s can be installed in a Mac Pro 1,1
- You can run three DVI displays on a single Radeon 5770
- Dual-Link Active Adapters are not needed for three displays
http://www.youtube.com/watch?v=rgbnE_NVuVY