First impression is that you aren't leaving enough space between the edges of the spine where you will surely lose the detail on the left and right hand sides due to the slight curve in the fold. Unless you're specifically going for that look I'd suggest leaving a space of approximately 2mm on either side (or 1mm if you're feeling the tight positioning).
Other than that looking good. Obviously the header depends on what you add to it, but if it's just the Steam logo and logotype it should be solid.
Other than that looking good. Obviously the header depends on what you add to it, but if it's just the Steam logo and logotype it should be solid.