looks pretty good to me, nex. some thoughts:
1) i would make the text a bit smaller. leave a couple open pixels around it or else it will look crammed in there.
2) the borders on the item buttons are pretty thick, too. i'd swipe a pixel or 2 inside and out.
3) Menu Titles (ie, "Load Game" in the layering test image) needs to be moved down a couple pixels (and with smaller text as mentioned).
as far as the transparency, i think it looks pretty slick but seeing it in action might be a different story. maybe leave the window with focus as you have it now but any other windows you could darken or fade more which would help make the text in top-most window more visible (ie, consider the overall contrast so (current) things stay legible).