For PiStorm there are several tutorials available on how to set up RTG, and the needed videocore drivers are available.
I have an A1200 with pistorm as well, but this is a different thing as the A1200 PiStorm will use the native video port for PAL/NTSC output and the Pi HDMI port for RTG. The team is working on a solution on how to get native video redirected to the HDMI.
On the A600GS both native and RTG will go out on the same HDMI output.
Why RTG? To get Workbench and apps to run on higher resolution and more colours, or run RTG games.