The pump/res placement is more or less irrelevant, though convention puts it before the cpu, for ease of tubing I've seen loops start with the gpu without issue.
The only thing I'd try to make sure of, is that you have a radiator inbetween each of the components. That way hot water from the cpu won't be fed directly into the gpu (or vica versa). I played around with my setup, and there really is a 2-3 degree difference. You'd be better off just doing whatever keeps the tubing routes cleanest/shortest.