Know Your Limits
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As much as you may want to, it's probable that your computer just can't push it to the limit in terms of graphical fidelity. Setting up your graphics configuration outside of what your system is capable of is a surefire way to get choppy framerates that ruin the experience.
This doesn't just apply to low and mid-tier machines though. Knowing the limitations of your rig can help the performance of any system.
Knowing whether your GPU is compatible with certain pixel shaders or levels of anti-aliasing can reduce a lot of strain on the system. This is most prevelant when considering vsync settings, which can really slow down older cards.