Depending on how much ram you have you may need to increase the voltage to the northbridge (the memory controller) slighty so that it can cope with the added stress of more memory. Before doing that though it would be best to check out the new memory to ensure its not that which is faulty.
Two ways of doing that, the simple (not 100% accurate method) and the correct but harder.
The first is to run your computer with just the new memory and see how that goes. Make sure you instal it in the same slots as you had the original memory in.
The second is to download
Memtest (
Memtest86+ - Advanced Memory Diagnostic Tool). To run that you'll need to burn the program to a CD and set your pc to boot from CD first. It will run automatically when you re-boot your pc and allow it to run it for a few complete passes. It is very important to test each single bank of memory individually...that way you can find out which, if any, are faulty.