3d Mark Vantage runs various tests of your system for the CPU and graphics. Once complete it gives you an overall score and a score for the CPU and GPU.

If you are overclocking then you can see how much 'improvement' you have made over the stock setup. I dont use it for anything else as most major overclockers will have vastly superior scores but its a good guide.

I've seen people use Superpi to test their computers performance but I found that crashed my system when every other test worked ok so I dont use it any more.