Diagnosing iMac hard crashes OS X 10.7.x, (updated to 10.7.2 when it became available last week); Quad Core iMac.

Hard crashing about every 2 or 3 weeks, except had one yesterday and another today, 5 times total so far. Mouse and keyboard go dead.  Screen frozen. Doesn't respond to LAN ping.  Dead.

Reboots fine.  Nothing at all about the crashes in the console all-message log.  No evident problems, operating normally.

Note: This machine has had no kernel panics at all in about a year, since I bought it, by the way.

My two suspects are 


Add-in-RAM -- maybe I should have bought the most expensive instead
of moderate-priced 4GB modules , a few weeks before the first
crash.
Firefox 7.0.1, as that was the active app when the last 2 crashes
occurred.


I'm keeping notes on the date, time, and front-most app when the crashes occur.

Any ideas? Any instrumentation to add?  Tests to run? (Given the time between crashes, I doubt a memory diagnostic is going to find anything.)
You could try testing the hard drive with Smart Utility, but it sounds like to me that it is your Ram that you installed. I have personal experience with bad/incompatible ram and the symptoms you are describing are exactly what happens if the ram is the aforementioned.