Fries your comp... nuff said~
Complete bullshit. If you know how to do it it can affect your performance in games, programs etc. Overclocking the CPU helps to get higher or more stable FPS. Overclocking the RAM will help you in benchmarks and overclocking the GPU will do the same as OC'ing the CPU.
Just make sure you have the right cooling solution you can push your PC to it "limits".