I think your avatar would be much better if you ditched the dithering, the only place I would even consider using it would be on the darkest blue background shades. I don't like the black outlines. I think the small metal bit under the orb should curve with the shape of the staff, it just ends in a straight line right now. I'd also advise using your darker blues to AA the swirlies on the border and floating around.
As for Excalibur, use a larger brush to start, lots of little small strokes makes the basic forms very hard to pick out.