On mine, this is a bucket size problem. 1st rest your dpi (waste as sam said), and cut your bucket in half until it works (32/16/8...). Stretches rendertime but gets the render. Ctrl/alt/delete during a render will show you if you've exceeded your RAM. Find a # that renders and try to stay below it. This should be a starting point and work your way up. Bucket size first. Maybe shadingrate(0.500 start) rate next. Or RayTrace if you're using it....one at a time. Focal is one of the toughest, so use last...all, imo Textures can be reduced in photoshop, too...