It is not easy to get hold of the T7400 2.16Ghz like you have mayer but if you find one I think they are the best value speed/cpu temperature. Newegg does not carry them any more at least. Ebay is a good source.
Long back in 2007 I upgraded my Mac Mini Solo to a 2.0Ghz Core Duo (Not Core 2) because it was really cheap. The pins on the Core Duo and the Core 2 Duo are the same also for T7400 and T7600. I was very satisfied with my Core Duo 2Ghz upgrade but my friends T7600 2,33Ghz was always running very hot. I think it is very important to use thermal paste like Arctic Silver and use very little paste. I would say '''too much is worse ''' than too little. The Mac Mini does not have very good cooling structure. I got my MacBook Pro (2007) back from Apple for a free logic board replacement. After that it was always running hot. So after some advice I took it apart and removed all the paste from the CPU sink. I used then only enough Arctic Silver to cover the contact area before putting it together again. Since it has run a lot cooler!
So you want a fast Mac Mini but not a hot one! We have enough fans blowing around us :-)