How much space should I leave between components? Reason I ask is that the HR21-200 and the TiVo HD are slightly less than 2U. If I want more than one quarter of an inch clearance between the top of them and the shelf for the next item, I'd have to move it up another U, and I have a 16U rack - it can get full fast. If I am fine with .25 inch clearance - I'll save 2U (one for DVR, and one for TiVo HD). However if they are too close they might overheat and cause me problems. Any thoughts or experiences?