Finally able to get my Radeon HD5870 1GB working with my HackPro 4.1.
No more issues with 2560×1600 requiring the display to sleep before it works. Also, all three monitors are working as well! 2 x DVI + 1 x DisplayPort to DVI (using an active adapter).
Steps are as follows (replicated from the InsanelyMac post):
1. Install all kext’s found on netkas’s post, EXCLUDING ATY_init.kext
2. Grab your ATI ROM and stick it in /Extras, naming it properly
3. Install the new boot7 loader
4. Boot with flags UseAtiRom=Yes GraphicsEnabler=Yes