[arch-general] Xorg doesn't start

[arno]

Hi,

I am install archlinux on my laptop, and I can't get my graphical card to work (Xorg doesn't start).

My card is an ati hd 6750

I've installed xf86-video-ati, xorg-xserver and xorg-xinit.
But when I try to run startx, xorg doesn't start, and I get a fatal error: no screens found. Here is the full Xorg log:
http://renevier.net/tmp/Xorg.0.log.txt

I tried to use catalyst as described on the wiki, but when trying to run aticonfig, I get the following error:
no supported adapters detected

lspci -v gives me:

01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Whistler [Radeon HD 6630M/6650M/6750M/7670M/7690M] (rev ff) (prog-if ff)
	!!! Unknown header type 7f
	Kernel driver in use: radeon
	Kernel modules: radeon, fglrx

I also need to mention that when I boot from the install cd, then mount my archlinux system and chroot into it, lspci -v gives me:

01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Whistler [Radeon HD 6630M/6650M/6750M/7670M/7690M] (prog-if 00 [VGA controller])
	Subsystem: Samsung Electronics Co Ltd Radeon HD 6750M
	Flags: bus master, fast devsel, latency 0, IRQ 51
	Memory at a0000000 (64-bit, prefetchable) [size=256M]
	Memory at c0700000 (64-bit, non-prefetchable) [size=128K]
	I/O ports at 3000 [size=256]
	Expansion ROM at c0720000 [disabled] [size=128K]
	Capabilities: [50] Power Management version 3
	Capabilities: [58] Express Legacy Endpoint, MSI 00
	Capabilities: [a0] MSI: Enable+ Count=1/1 Maskable- 64bit+
	Capabilities: [100] Vendor Specific Information: ID=0001 Rev=1 Len=010 <?>
	Capabilities: [150] Advanced Error Reporting
	Kernel driver in use: radeon

So this brings me to my first subquestion: why does lspci work fine when booting from the install/repair CD, but displays "Unknown header type" when booting from my installed system?

And my main question is of course: what could I do to get my card working, and X starting?

thanks a lot.