I was wondering if it would be possible to load 2 different xorg.conf files based on session selection? E.g. xorg.conf_1 with kde session, xorg.conf_2 with xbmc session.

I have an ATI built-in onboard adapter ( I would use it for XBMC) and
I have a NVidia for the desktop use. I found no way to load both drivers at the same time.

So I wonder if I load one of them with a Xsession.

Any good optional ideas appreciated!

Thanks in advance,

This would involve a complete restart of the X server, which does not happen when you login. However, you could use other ways to do this ( such as having two virtual terminals, changed between using Ctrl + Alt + F7 / F8 with differently configured X servers running on each )

Thanks bcooksley!

So my idea is just not possible

Your advice looks reasonable, however it takes some time to figure out how. I am not so advanced user of linux.

I do not know if two instances of X server will use up CPU and memory. Would that be possible to run only one X and switch with a hotkey?

My second idea was to use runlevels. E.g. runlevel2 for XBMC with ATI card and runlevel3 for normal desktop use with NVIDIA card. Does runlevel still exist in kubuntu 10.04 ? I have not found /etc/inittab. Would this work instead?

Runlevels could possibly work too, yes.
Running two X instances will use more memory.

Switching with a hot key may not be possible... unless you could get it to fire off a script to shut down the current X session, then start it with the other X.