rtg stands for ReTargetable Graphics... you need an amiga graphics card (installed in a tower with some zorro/pci slots or use a BlizzardPPC+Bvision card) then you will be able to use resolotions like 800X600,1024X768,... @ 8bit/16bit/24bit and all these without wasting chipmem, or speed... the graphics card takes care of all the operations, and everything is much faster!
I currently use a 800X600@16bit resolution, and everything is much nicer and FAST...