On Monday I successfully upgraded the CPU of my Mac Mini. The old Intel Core Solo 1,5 Ghz had to give way for a new Intel Core 2 Duo 2,0 Ghz (T7200, code name “Merom”). Additionally I replaced the two 512 MB RAM modules with two modules of 1024 MB each.
1. The Mac Mini still works stable! 
2. The speedup is really huge. I did not do many exact benchmarks but most applications are starting much faster than before (espcially iPhoto with 8000 photos and Rosetta applications like MS Office or Acrobat 7) and feel more responsive.
3. The CPU temperature during normal usage (running apps: AdiumX, Skype, Mail, NetNewsWire, iTunes, EyeTV) ranges from 55 to 67 degrees celsius. At maximum load the temperature went up to 97 degrees for a short time (most of the time it was at 90 to 92 when running SETI@home). Although this is within the limit of critical temperature (which is at 125 degrees celsius for the Core 2 Duo), it is unfortunately not very low. I did not measure the temperature of the Core Solo 1,5 Ghz before the upgrade, but can remember that I ran CoreDuoTemp some weeks ago during normal system usage and got about 60 degrees celsius.
4. Some images of the upgrade can be found here.
Benchmarks:
Here are some benchmark values of the Mac Mini with the old CPU (first value) and the new one (second value). The third value shows the speedup-factor.
Xbench values
CPU Test: 56.20 -> 104.55 => 1,86
Thread Test: 82.82 -> 199.50 => 2,40
Memory Test: 105.20 -> 117.77 => 1,12
Most of the other Xbench tests showed a speedup similar to the memory test.
Cinebench 9.5
Rendering (Single CPU): 216 -> 332 => 1,54
Rendering (Multiple CPU): n/a -> 619
Multiprocessor Speedup of Core 2 Duo: 1.87
Shading (CINEMA 4D): 265 – 405 => 1,53
Shading (OpenGL Software Lighting): 850 -> 1440 => 1,69
Shading (OpenGL Hardware Lighting) : 869 -> 1574 => 1,81
OpenGL Speedup: 3.28 -> 3.88 => 1,19
Result: SPEEEEEED 
Have you thought of trying it with 3GB of RAM?
No.