it it possible to upgrade the CPU, I have seen several reports of this
for example see [http://www.macintouch.com/specialreports/minimonster/|this article] discussing the upgrade of a Core Solo machine to a Core 2 Duo CPU.
which version of Mac mini do you have?
you can also find various user reports of upgrades like this over at [http://forums.xlr8yourmac.com/cpureview.lasso|xlr8yourmac.com] - just enter your Mac model in the top popup, and select "Intel CPU upgrade" in the 2nd popup