I had the very same problem ("PreInitDAL failed", after EDID data), with my new Sapphire Radeon HD 3870 (PCI-Express).

I was using uvesafb. After seeing http://gentoo-wiki.com/HOWTO_ATI_Dri...InitDAL_failed, I deactivated it (at boot... no need to remove it from the kernel). It now works without problem (as per `glxinfo` and `glxgears`).
Well, I don't think that deactivating the framebuffer will solve the problem [for all]... I tried that too (vesafb), but I wasn't lucky... It continued without working...

Snake, that's true, its not AGP-specific, my HD2400 is for pcie...

I'll add the output of /var/log/Xorg.0.log later, please tell me if it is necessary to post any other files...

Thanks in advance for all your help...