Having never heard of GART or GART memory the suggestion in the error message did not mean a lot to me, however with a little research I found an option in the file:
/etc/X11/xorg.conf
Within the "Device" section of the file I added the line:
Option "GARTSize" "64"
Having never heard of GART or GART memory the suggestion in the error message did not mean a lot to me, however with a little research I found an option in the file:
/etc/X11/xorg.conf
Within the "Device" section of the file I added the line:
Option "GARTSize" "64"
Re-booted and Stellarium now works OK.
My PC has 4 GBytes RAM and an ATI X300 video card
I hope this helps.