This past year and a half I got 2 Pre-'09 iMac Mini of the Intel Variety because obsolescence of the G4/G5 processor is catching up to me.  One was a Core Single and the other was a Core Duo, the Single was upgraded to a Core 2 Duo with a simple upgrade with a CPU Swap.
 
IFixIt.com's as an almost complete instructions on how to do this upgrade. While you are in there, upgrade the RAM to 2GB and a larger and faster hard drive or SSD. Here's IFixIt's Mac Mini Upgrade webpage; I'm surprised on how complete it is:
https://www.ifixit.com/Guide/Installing-Mac-mini-Model-A1176-Core-2-Duo-Processo r/1178/1
 
After the upgrade, you need to put the Mac Mini in FireWire Mode and delete the hidden file: /System/Library/CoreServices/PlatformSupport.plist
 
Then you are reboot the system and it will run better than before.
 
With the Core 2 Duo, you can hack/upgrade the Mac Mini's OS to the current level of El Capitan (OSX 10.10) and Yosimite (OSX 10.11). With the Core Single or Core Duo, you are stuck with OSX 10.7.
 
I was looking into swapping out the Core Dup and Core 2 Dup with a Core 2 Quad, but the CPU is different, many desktops and servers use a Socket 775 for the Core-series CPUs while the Mac Mini and laptops use the Socket-M (Mobile) 479. The Core 2 Quad was never made as a 479-mobile CPU.
 
Considering how prices are dropping for the Mac Mini, one should get a Pre '09 Mac Mini of Single or Core Duo and do the upgrade. On and after '09, the Mac Mini can not be given this upgrade; the '05 to '09 Mac Mini's can but beware - '05 was the last year for the G4 Mac Mini, do not confuse it for the Intel Mac Mini of the same year.
 
BTW - The G4 Mac Mini prices are dropping through the floor and into the basement, I'm seeing them for under $50 on ebay. Though their upgrade options are limited, they are great for many Mac things. If you get one, you should upgrade the RAM to its maximum of 1GB and the hard drive to the biggest and fastest IDE/PATA or IDE/PATA SSD you can find. Though it can run OSX 10.5, I recommend OSX 10.4 in order to use older 68K/PowerPC software while running Mac OS9 on top of OSX. 10.5 will not let you access OS9.