Well, the operation was a success.

The 3Ghz chips were actually the X5472 version (120W vs 80W) and work a treat. They run slightly warmer, bt not by much, only about 3 degrees.
To minimise the heat gain I lapped the top of the chips with 1200grit paper before I cleaned and applied AS5 thermal paste.This is to smooth them out for optimum contact with the heatsink.
Mac sees the chips fine and reports them correctly, as I'd expect as these machine used to ship with these under the 3Ghz version.
Overall a 7% gain in multi-processor calcs, and 8% gain in OpenGL performance.
Overall took 30mins to upgrade...Not bad.
Next upgrade is a GFX card.
Jacques
Some photos here on Facebook.
http://www.facebook.com/media/set/?s...p;l=983be7de35