Hi
I was called out to an old customer of mine once, who had just bought a sort of wardrobe to sit in their hall and totally contain their whole computer system...
It was cutting out from overheating in UNDER 5 minutes from cold...
When I got there and opened the twin doors, the pc tower was squeezed into a space at the bottom that was 1/4 of an inch bigger than the tower....thats 1/4 at the top and both sides and the back was totally blocked off...
I could do nothing apart from remove the tower and stand it beside the unit... and voila....NO overheating and NO cutting out...at all...
So yes you need ventilation and I dont mean fans etc inside the tower...I mean OUTSIDE THE TOWER...
Hope this helps...
All the best Brett