Monitoring Your Video Card
In addition to testing stability, it is equally important to closely monitor your video card. There are several good programs to do this.
GPU-Z: The most popular and free simple graphics card information and monitoring software. It is great for certifying clock speeds, checking your video card, monitoring temperature, and learning more than you would ever need to know about your card.
RivaTuner: The most popular overclocking tool for graphics cards. It also has a nice GUI and temperature monitoring application for checking clock speeds, temperatures, and fan speeds.
ATI Catalyst Control Center Driver Package: This is included in all ATI driver suites (CCC). Under the OverDrive tab, is a monitor for clock speeds, temperatures, and fan speeds. You can use this to tune your video card. Again, this will only work for ATI cards.
As far as I know, Nvidia does not have any monitoring built into their drivers. If you know differently, please let us know in the comments!