My laptop has an ATI Mobility Radeon X1600 and even with the latest fglrx drivers as installed with SL 3.26, then SL 3.3 and now 3.4 loop 2b, hardware acceleration is a no-no: no DRI, no XGL (and no AIGLX).
And with SL 3.4 Loop 2b only one resolution (the maximum detected, viz. 1680 x 1050 for my laptop) is available in Control Centre > Peripherals > Display Screen size. It does not matter what I do with xorg.conf, only one screen size is available. So I need to have to have two versions of xorg.conf: one when I'm using my laptop by itself and another when I'm using it with and external monitor. With SL 3.26 and 3.3 I just needed to adjust the screen size from the Control Centre.
Seems ATI is not SL-friendly.