go to 'file/options/display options/, tick the 'enable background draw' & turn the sliders up to at least halfway, then click 'ok'...u should then see the plants/objects instead of the boxes,,if u don't see plants/objects instead of boxes, turn the sliders up more...this will use more resources,tho.

as for the max useable mem, my gc card is 2 GB, so I set mine @2000..but if your card is 512, then you shouldn't go over that amount..