Don't forget that as you are going to re-render to a different aspect ratio, that the image may not show all of what you want. 800x600 is a 4:3 aspect ratio, while 3000x2000 is a 3:2 aspect ratio. Do a test render at say, 600x400 (same ratio as your final size) to see if there is a noticable difference in what will be in your final pic. Not related to the question as such (viewing the rendering), but still fairly important.