It will also buy you 3-5 degrees better performance over the Arctic Silver.

This is not correct, at least not for air coolers. MX-2 is preferred in liquid and phase-change cooling applications where condensation is a concern. This review demonstrates that AC5 is still among the best TIMs on the market. This review also demonstrates the same fact. You'll notice MX-2 and AC5 are compared, with AC5 coming out on top in both reviews.As for which CPU stress test generates the most heat: nothing beats Intel Burn Test (also works on AMD CPUs). I personally run OCCT as a stress-tester. It runs a few degrees cooler than Intel Burn Test, but it's the next-best-thing and it will find faults within a few minutes, generally. That certainly beats several hours ;)

That's exactly the problem. I can run my system pretty nicely at even 4Ghz but for some reason the only software that does not get along with the overclocking is FSX.

Well,I doubt that the problem is Fs9 or fsX.I am running an INTEL E8500 (standard at 3.16) on a gigabyte motherboard (GA-EX38T-DQ6) and it runs happily at 4.45 Ghz. In fact, I can even push it a little bit farther up then I am running into trouble with the settings of my memory. (which is Corsair Dominator twin3x2048-1800c7df).The main rtouble with overclocking is getting it all in sync with eachother. Pushing the processor is one thing but if the timings of your memory are off, you will not get far up the Mhz-ladder. Another bottleneck is probably 'power'. You need a very stable powersupply and you need (manual or regulated by the bios) to adjust the voltages.I am not a wizard when it comes to overclocking. I have read the manual that was supplied with the motherboard and I let my bios run the difficult parts (like current and memorysettings).At 4.45 Ghz both Fs-9 ans Fs-X run stable but as one of the posters already mentionned, every system runs different.Luc Brusselmans