The first thing to do is to test the new memory very thoroughly. The first ram test tool I found on the net is at:
http://www.memtest86.com/It's GPL'ed - and comes as an iso so you can burn it (from another computer), boot from it and if it's anything like the one I use it'll run for about an hour or more, the longer the better, and do a pretty thorough test. If it throws up errors, try re-fitting the memory first of all.
Best Wishes, Max