Hi,
What ever the resolution you are using, Atomic will adapt the Layout for you. (by strechting). So you can use a 640x480 layout in 800x600 resolution. (but there is no real interrest to use a 640x480 layout in 800x600 because it won't be nicer , just consume more memory).
Or you can try to adapt yourself the layout to 800x600 , open it with AtomicLayoutEditor , go to layout\properties , change the resolution to 800x600 , resize or change image you need and adjust fonts as you want.