May as well get a core 2 duo, if you are going to bother. Unless the price on the T2600 is very attractive.
I "upgraded" my core duo 1.66 with a core 2 duo 1.66 (got a good deal, I wouldn't recommend it as an upgrade path), and it netted between a 5-10% increase on some of the benchmark scores.
As far as the speed increase - over what? A core solo? Duo 1.66? 1.83?
Since the mini uses old macbook parts, it would make sense that when the macbook goes santa rosa. All the left over C2D chips would make it over to the Mini.
Pulled the T2300 out of my mini very shortly after purchase and swapped in a T7200.
It was- and still is, in my opinion- the best bang-for-the-buck CPU upgrade option.
Best of luck!
Have you noticed any changes in the temp of the computer, now you are using the Core 2?
I can't really speak to any differences, because I didn't take any baseline readings on the original CPU.
But perhaps this (grabbed at idle) will help:
Hey that doesn't get too much hotter. Hower I am assuming that you here not doing any thing. What about the fan noise? As it goten louder?