For CPU/RAM stability, Orthos is probably the best - you need to make sure all CPU cores are stressed at the same time. Your system technically needs to pass small FFTs, blend, and large in-place FFTs. Small FFTs tests core logic, large FFTs tests either L2 cache (Core 2) or RAM (A64), and blend tests CPU, cache, and RAM all at once. You're pretty safe if you can run all three tests for an hour at worst-case ambient temperature.
For a gaming machine, especially where the graphics card does not vent out to the back, one should also repeat Orthos at lower priority while running 3Dmark in a loop to see if there are any heat-related issues.