I design embedded PC's for a living, and the CPU is not the critical part. An Intel CPU will get to 100C before it starts shutting down. The DRAM and HDD are your weak points. Standard DRAM will go to 65 or so, which at room temp is pretty easy to get exceed with a poorly designed cooling solution. The HDD will also have a max case temp of 60, which again is pretty easy to get to.
I have designed fanless dual-core industrial PC's that will run at 55C AMBIENT temp (in a thermal chamber) with industrial temp HDD's and DRAM. The CPU is running right at 100 and the DRAM is at 85, so these things get very toasty, but they run fine. The case on these things are normally a solid heatsink and at 55C in the chamber they are normally too hot to pickup without gloves! We cook these for 48 hours at 80% CPU utilisation on each core and everything else running flat out (Ethernet, memory etc).
That's why I chuckle when I see postings about 40C CPU on-die temps....
- Tim