In summary, the AMD driver will not work with open SUSE-12.3 with your older Radeon Xpress 1150.There is no update to the open source radeon driver (for open SUSE) that I am aware of.
However to optimize for your hardware likely requires specific knowledge from someone with identical old hardware who has taken the time to research this and test this. I simply do not know and can not answer your question whether it is possible to better tune the radeon drivers performance.This was my next step anyway, to dive into the OS and start tweaking, but I wanted to make sure I covered all my bases with hardware first and got all that works as best I could. when I format a new computer with windows, the first thing I do is update all the drivers and then I start tweaking and customizing windows. I recall asking if you were certain the radeon graphic driver is being used on your PC and not the vesa nor fbdev driver ?Have I done all I can for the 'update all drivers' phase with Open SUSE? I can not say 'yes' to your question without mine being answered.The link noted by Sauerland gives a good summary of the AMD support for GNU/Linux (and the versions of the kernal and the versions of X).Your older Radeon Xpress 1150 is not supported by a recent AMD proprietary driver and it definitely will not work with the latest kernels nor with the latest versions of X.