I recently upgraded our office Mac Mini from 1GB of RAM to 2GB. Afterwards I was having a number of issues with the machine. Event with Macs using the Intel chipset there are still a number of differences in the system and startup procedures. The Mac has a number of system controllers that cache settings, and occasionally become corrupt. In the PC world, we don't have to worry about this, so it doesn't come directly to mind that you have to reset these controllers after making changes.
Resetting the Mac Mini:
Parameter RAM (PRAM) and Nonvolatile RAM (NVRAM): Hold down Option-Apple-P-R while turning the power on. Continue to hold until you hear the start sound twice. http://support.apple.com/kb/HT1379
Symptoms of corrupted PRAM/NVRAM - System unresponsive, Will not go past the Apple Load screen, frequent crashes or video problems.
System Management Controller (SMC): Unplug all the cables (power, video, keyboard, etc.) from the mini. Wait 15 seconds plug all cables back in. DO NOT Hold down the power button while plugging in the power cable. Turn on the mini. http://support.apple.com/kb/HT1543
Symptoms of corrupted SMC: Fan runs constantly and loudly. Performance problems. Computer sleep problems.
Power Management Unit (PMU) on older minis: Unplug all the cables (power, video, keyboard, etc.) from the mini. Wait 10 seconds plug all cables but the power cable back in. Hold down the power button while plugging in the power cable. Release the power button. Turn on the mini http://support.apple.com/kb/HT2183
Symptoms of corrupted PMU. System won't start, peripherals won't work.