One of the best ways to determine if your system is stable is to download Prime95 and run a torture test on it (I think
this is their webpage but it's down right now). This will make your computer preform some very complex math problems and then verify the answers against known results. It will also heat your cpu up more than Sandra. When you first start the torture test you may want to wiggle the mouse around every once in a while to make sure your system hasn't locked up (usually caused by heat or not having enough voltage in my experience).
Some people like to leave the program running for as long as 48 hour to verify stability, but just running it overnight is usually good enough for me.
