040,PPC and BV in a desktop-case is the worst setup you can get when it comes to heat problems. You may need to put the Amiga on feets, and surely you'll need to leave the trapdoor open.
All this will reduce heat-relted probs, but it won't make them go away...
The 060 on the other side should just work without any probs and will give you much more speed for everday use and whload compared to the lousy 040.
Diplay speed will offcourse be slower, but you will be saved from switching everytime you start a game.
Dunno how much difference their is in price for you, but IMO a PPC would just be overkill for what ou plan.
