De-dusting my computers

Yesterday, after revealing my concerns about high CPU operating temperatures, I considered opening up the computer and removing any dust in an attempt to get the processor temperatures down.
So, today I opened up my Dell E520 and was surprised to find very little visible dust inside even though it's been about a year since I last took off the cover.
The gridded air inlet at the front of the box was the most dist-laden with maybe 40% of the grid holes apparently blocked.
Amazingly, there were quite a few very wispy spiders webs within the computer although I can't imagine these had any effects on the cooling.
What did the spiders hope to catch in there?
Neither the CPU nor the graphics card has a fan but both have sizable heat sinks.
Anyway I spent about 10 minutes vacuuming with a narrow nozzle (high air velocity) in every possible location where there might have been dust even if I didn't see it.
Then I put everything back together and fired up FreeBSD 8.2.
Before the cleaning, I typically had 62-64ºC in the processor and 72-74ºC in the GPU at full CPU frequency (1.87 GHz) and some moderate operation (browsing, for example).
When completely idle and using powerd (cpufreq varies between 0.8-1.4 GHz) temperatures dropped to 48-52ºC in the processor and 64-66ºC in the graphics card.
After dedusting, the equivalent idling temperatures plummeted to 39-42ºC in the CPU and 60-61ºC in the GPU.
That's quite an improvement.
With moderate processor use (browsing) but full CPU frequency, CPU temperature rose to 44ºC while the GPU temperature increased to about 63ºC.
Again, a very appreciable improvement even though very little visible dust was evident.
I subsequently ran a "portupgrade -a" command which involves very long periods of very high CPU (75-100%) CPU operation.
Unfortunately, I neglected to run this before the clean to make the comparison.
Nevertheless, the figures are interesting.
Typical CPU temperatures during this operation were 60-62ºC while GPU ran at 65ºC;
The absolute highest individual temperatures I saw were 63ºC for CPU core0, 62ºC for CPU core1 and 66ºC for the GPU.
These compare with highest equivalent values before cleaning of 66ºC for core0, 67ºC for core1 and 76ºC in the GPU and we must remember that these latter values do not at all compare to extreme (100% CPU) conditions.
I would therefore be prepared to say that removing the dust from inside the computer reduced CPU temperatures by 10-15ºC and that in the GPU by 8-12ºC.
Nevertheless, it's worth noting that the CPU temperatures still frequently exceed the stated critical temperature for this processor of 61.4ºC.

Just for comparison, I installed FreeBSD 8,2 on another machine (lightly used Dell Dimension 9200 with nVidia GeForce 210 graphics and Intel Core2 6400@2.13GHz) and tried the same tests before and after dedusting.
Once again, very little dust visible inside the computer although, once again, it was most visible on the air inlet grid.
This GPU has a fan but the processor is again fanless and heat sinked.
The most striking differences were that both CPU and graphics card temperatures were much lower in this machine. While the GPU improvement may be explained by the fan, it is not clear to me why the CPU temperatures are so much lower.
I'll cite the results in three situations equivalent to those for the E520 box, that is, idling, moderate load and full throttle (the very same "portupgrade -a") and before and after cleaning results will be given:
The results order are CPU0, CPU1 and GPU.

1. Idling before dedust: 25ºC, 25ºC, 40ºC
2. Idling after dedust: 24ºC, 25ºC, 40ºC
3. Moderate load before dedust: 27ºC, 27ºC, 40ºC
4. Moderate load after dedust: 27ºC, 27ºC, 40ºC
5. Heavy load before dedust:
6. Heavy load after dedust: 41ºC, 41ºC, 41ºC

With such a huge contrast in these results between the two machines, I need to see if I have a failed fan somewhere in the E520.