If you want to know absolutely everything there is to know about your hardware--but you don't want to spend any money on software to do it for you--you'd do well to download the free HWiNFO32. This freebie provides an astonishing amount of information about every aspect of your hardware, and it's extremely easy to use.
HWiNFO32's single best screen is the one that shows a system summary. You'll get details about your CPU, including all cores if it's multiple core, your graphics card, BIOS, RAM, system cache, drives, and more. It's more information you might ever want.
In addition, HWiNFO32 cans show you separate screens with even more information about each individual component in your PC, all easily browsable. You can also run benchmarks on your hardware and see now just the raw performance numbers, but how your hardware stacks up against other systems.
Even more useful are the HWiNFO32's sensors, which report on the temperature of your CPU, core by core, your graphics card, and your hard disks. That way, you can see whether any are overheating, and forestall any problems by seeking help before damage is done.
Note that if you use Windows Vista, you'll have to run HWiNFO32 as an administrator. Right-click the executable and select Run as administrator from the menu that appears in order to do so.