Pre Workout Amplifiers?

I want to increase my lifts and results in the gym. I have progressed alot over the past two years and I want to take my results to a whole new level. I've looked into pre workout amplifiers like SP250, Allmax Muscle Prime, N.O Shotgun, Xyience Nitric Oxide etc... I understand their purpose, but what are some side effects? My circulatory and cardio are in top notch shape and I dont want to ruin that. A little enlightenment please?