I took a friends old cabinet since he's moving out of town, and my old one was pathetic. The problem I have is that when I open the glass, I notice that it is very hot inside of there. Should I put a fan inside of there, and if so, are there any fans specifically for a HT that might have less interference than others?

<img src="http://homepage.mac.com/ashook/awot/hometheater.jpg">
<img src="http://homepage.mac.com/ashook/awot/hometheater2.jpg">

I once owned a cabinet where the backing fell off, and it was very shaky afterwards.

Tons of ventilation too

shelf in the cabinet and then proceed to cut out only the top portion of the rear panel(leaving the edges or corners in place for rigidity)

1.The two hottest items - the DVD player and the amp are together - they should be seperate. I would move the DVDs and tapes somewhere else.
2.You could drill holes in the back - several one inch holes for each shelf should not weaken the back support.
3. you can do a google search for fans - they will have to exit the back with their exhaust - and computer case fans would work too - if large enough.

with a drill?

:-P