Having got the HTPC working as an after thought I have tested the watts drawn at the wall.
The AMD 4850e solution, with the same WD Green power drive would draw around 80 watts idle and a little more in normal use, peaking up to 120w.
This solution, with the addition of an SSD drive and faster RAM peaked at 42 watts during boot and 28 watts idle - much better, I've since added the Hitachi 1TB drive so that will raise that a little.
I have covered all of the vents with old tights with the exception of the top vent (on the thought that heat is rising so lifting dust away, that will get reviewed in a couple of months), affixed inside on the sides and outside on the bottom, the finished product is trimmed neatly and not visible in the cabinet at all. I used this trick on the Silverstone case just to reduce the amount of dust that is able to enter the case while not stopping sensible airflow. It is not a perfect solution, tights do not make a perfect seal (not helped by wife's unwillingness to donate a pair without ladders, something about wanting to wear them) and there are plenty of gaps in the average case that let some air in anyway but any reduction will reduce maintenance required.