I still don't know how much clearance is in there. I agree, cutting a hole in the top of the case for more ventilation would be ideal. But, as I'm using this as a HTPC, it sits idle 99% of the time, and temps are reasonable. And I don't trust my aluminum cutting skills enough to cut a hole and keep it aesthetically pleasing. At all times, the case is actually quite warm to the touch; not hot, but noticeably warm. But I whipped up a quick script to log temperatures, and even when we are watching movies, temps are fine. And I have the fan dialed down in the BIOS, so it's effectively silent (can only hear the faintest "whooshing" when heat is one foot away or closer).
Having said that, I'm sure the components would not survive if this thing was running at any significant CPU load for long lengths of time. A few weekends ago, I walked right past where this HTPC sits, and thought I smelled a faint burning odor. I had just completed the building of a pair of DIY speakers which were also setting in the same vicinity, so I attributed the smell to the speakers. That night, we watched a movie or something on the HTPC, and there was some random freezes/pauses in the video; that never
happens, so when the show was over, I walked over to the unit to pull out the keyboard and start looking for rogue processes or something on the box... several
programs, including the MythTV backend process, firefox, and a few others were all consuming 100% CPU. The Big Shuriken fan had ramped up to full speed, as was quite audible. CPU temperature was over 90 degrees Celsius. And the case was too hot to even touch! I didn't know what was wrong, so I rebooted. When the system came back, all processes were normal, but I kept the fan on full speed to cool it off. When the case was safe to touch, I removed the top plate to speed up cooling.
I later found out that day had a "leap second". The Linux kernel's handling of this was buggy, and it caused several programs to go haywire and jump to 100% CPU load. My temperature logger indicated that right at 7:00pm CDT is when it happened, and I didn't address the issue until midnight or so!
In hindsight, I don't know why I thought my speakers would have emitted a burning smell (they were from a kit where all I did was glue and screw, didn't even use any paint or stain). The system has worked just fine ever since... but I may have shortened the life of some of those components; only time will tell.