Good idea, I think im going to do this on my server box later today. Theres not enough room to mount it on the inside but thats okay seeing as its under my desk anyhow.
Stick an old heatsink off of an old video card/chipset on there if you can figure a way to mount it (doesn't have to be on superhard, just enough to draw some heat away) and it will help a lot.
tell you what Ill keep reading and you start to think a bit.
If there IS space between the motherboard and the case side (i.e. motherboard tray) it wont be much, so he would need a slimmer fan. If not, he would stillw ant to cut a hole in the side for the air to escape, and why woudl you want somethign sticking really far out of the side of the case. I would say that the less obtrusive the mod is, the better.
