What would be the minimum top ventilation requirementments for a 100w/channel receiver?
I currently have three units stacked on a stand with no shelves:

RECEIVER
DVD PLAYER
DVD PLAYER

Since the receiver is the only one with top vents, no problem. Got a good deal on a standalone cd recorder and was planning on slipping it between the top dvd player and the receiver but it turned out top have top vents too. If I put the recorder on top of receiver I have 3/4" ventilation for receiver but concerned this may not be enough. Thought maybe I could put something under the recorder's legs to in increase the space. What's a safe minimum?

Hmmmm. Well, I would suggest getting a simple shelf for your components. At least get the receiver away from the other ones. Just my opinion.

You should not have even a cheap receiver sitting on top of another component, they are too heavy and most DVD players are not meant to hold anything on top. You ought to be able to stack 3 disc players (since they are pretty light) if you can get the receiver its own shelf. It can be very close to the players, above them, but on a shelf. If the receiver was on the bottom, you'd need more ventilation space since heat goes up, but on top it can be fairly tight.

If you need cheap ways to separate 2 players with no shelf, try hockey pucks. Three of them, situated strategically on the edges.