I have a Sapphire 4850HD along with the following Gigabyte GA_P55M- UD2, Intel Core i7, 4 GB RAM, 1 TB Samsung HD and an Asus 23.5 " HD Display.

The Sapphire Videocard has been the sticking point. I have a working installation os OSX SL 10.6.2 that will boot via Chameleon in safe mode (-x).

Without editing the ATI4800 kext to include the device ID the card is recognized as such:

ATI Radeon 4850 Series:

Chipset Model: ATI Radeon 4850 Series
Type: GPU
Bus: PCIe
Slot: Slot-1
PCIe Lane Width: x16
VRAM (Total): 256 MB
Vendor: ATI (0x1002)
Device ID: 0x9442
Revision ID: 0x0000
ROM Revision: 113-B7710C-176
EFI Driver Version: 01.00.318
Resolution: 1024 x 768
Pixel Depth: 32-Bit Color (ARGB8888)
Main Display: Yes
Mirror: Off
Online: Yes
Display Connector:

Obviously its only seeing half of the cards ram. 256 not 512.

As per this thread I have tried modifiying the recommended kext file.
How to: ATI 4850 working on SL - InsanelyMac Forum

When I tried this (in safe and normal full boot modes) the monitor displayed an out of range error.

I was forced to remove the drive and return the ATI4800Controller.kext to its previous code. Then it would boot back into safe mode.

Next I tried the evoenabler.ket along with the modified string.
Same result. (Out of Range)

I am wondering if there is a way to manually choose a display resolution and frequency. In the early versions of OSX this was done by editing the windowsserver.plist

I'm aslo wondering if the recommended device ID being added to the ATI4800Controller.kext is correct for my card.

I have a feeling I am really close to having this work but someone with a little more experience or knowledge of Snow Leopard would be helpful.