I just bought a mac 9800xt card to replace my 9600pro on my G5 running gentoo ppc64 linux(2006.0, 64-bit userland). The system worked fine until I started xorg-7.0: the screen blacked and the keyboard and mouse were not responding any more, so I had to shutdown the machine. The same X config worked well with 9600pro card(64M).
I tried also ppc32 chroot on my system with xorg-6.8.2, it worked as expected. The last part of Xorg.0.log file is as follows, which was left by the broken xorg-7.0:

(==) RADEON(0): RGB weight 888
(II) RADEON(0): Using 8 bits per RGB (8 bit DAC)
(II) Loading sub module "fbdevhw"
(II) LoadModule: "fbdevhw"
(II) Reloading /usr/lib64/xorg/modules/linux/libfbdevhw.so
(**) RADEON(0): Using framebuffer device
(--) RADEON(0): Chipset: "ATI Radeon 9800PRO NH (AGP)" (ChipID = 0x4e48)
(--) RADEON(0): Linear framebuffer at 0xa0000000
(--) RADEON(0): BIOS at 0x90020000
(--) RADEON(0): VideoRAM: 262144 kByte (256 bit DDR SDRAM)
(II) RADEON(0): Color tiling disabled
(II) Loading sub module "ddc"
(II) LoadModule: "ddc"
(II) Loading /usr/lib64/xorg/modules/libddc.so
(II) Module ddc: vendor="X.Org Foundation"
compiled for 7.0.0, module version = 1.0.0
ABI class: X.Org Video Driver, version 0.8
(II) Loading sub module "i2c"
(II) LoadModule: "i2c"
(II) Loading /usr/lib64/xorg/modules/libi2c.so
(II) Module i2c: vendor="X.Org Foundation"
compiled for 7.0.0, module version = 1.2.0
ABI class: X.Org Video Driver, version 0.8
(II) RADEON(0): I2C bus "DDC" initialized.
(WW) RADEON(0): Video BIOS not detected in PCI space!
(WW) RADEON(0): Attempting to read Video BIOS from legacy ISA space!
(WW) RADEON(0): Unrecognized BIOS signature, BIOS data will not be used
(WW) RADEON(0): Invalid Monitor type specified for 2nd port
(**) RADEON(0): MonitorLayout Option:
Monitor1--Type TMDS, Monitor2--Type

(II) RADEON(0): I2C device "DDC:ddc2" registered at address 0xA0.
(II) RADEON(0): I2C device "DDC:ddc2" removed.
(II) RADEON(0): DDC Type: 2, Detected Type: 3

f0:10.0 VGA compatible controller: ATI Technologies Inc Radeon R350 [Radeon 9800 Pro] (prog-if 00 [VGA])
Subsystem: ATI Technologies Inc Radeon R350 [Radeon 9800 Pro]
Flags: bus master, stepping, 66MHz, medium devsel, latency 16, IRQ 48
Memory at a0000000 (32-bit, prefetchable) [size=256M]
I/O ports at f0000400 [size=256]
Memory at 90000000 (32-bit, non-prefetchable) [size=64K]
Expansion ROM at 90020000 [size=128K]
Capabilities: [58] AGP version 3.0
Capabilities: [50] Power Management version 2

X Window System Version 7.0.0
Release Date: 21 December 2005
X Protocol Version 11, Revision 0, Release 7.0
Build Operating System:Linux 2.6.16-ck2 ppc64
Current Operating System: Linux localhost 2.6.16-ck3 #1 SMP Mon Apr 3 19:12:29 CST 2006 ppc64
Build Date: 02 April 2006
Before reporting problems, check http://wiki.x.org
to make sure that you have the latest version.
Module Loader present

Section "Device"
Identifier "Card0"
### Available Driver options:
Option "ColorTiling" "false" # True has trouble with latest xorg
Option "BackingStore" "true" # Increases performance
Option "EnablePageFlip" "true" # Increases performance
#Option "SWcursor" "false" # Use Software Cursor
Option "NoAccel" "false" # Disable Hardware Acceleration
Option "AGPMode" "8" # Set AGP data transfer rate, DRI only
#Option "AccelMethod" "EXA" # New RenderAccel
#Option "AGPFastWrite" "false" # Use AGP Fast Write, DRI only
Option "RenderAccel" "true" # Hardware acceleration
#Option "DCCMode" "false" # Use querried modes from monitor
Option "MonitorLayout" "TMDS" # Enable this for Apple DVI LCDs
### See "man radeon" for additional options
Option "UseFBDev" "True" #"False"
Driver "radeon"
BusID "PCI:240:16:0"
EndSection
... ...