MacMini crashing after changing memory?
The Mac mini (mid 2010) is extremely finicky accepting RAM with a faster clock-speed (some work, some don't). You must use 1066 MHz or buy a pair that is known to be free of issue. For some reason that particular model was extremely sensitive to any discrepancy in RAM (many posts on the internet regarding its inability to run under these conditions).

For what it's worth, the new Mac mini (mid 2011) do not suffer from the same problem, accepting higher clocked RAM without complaint.