Summary & Conclusion
In the end, it doesn't necessarily matter how much you paid for your paste, or what thermal conductivity its manufacturer claims on the package. More important is the how: your application method makes or breaks this process.
Some of the pastes we tested remain classics, and are even relatively inexpensive. It only follows, then, that we'd caution you against spending big on a brand new paste that'll only make a temperature difference of 1 to 2 Kelvin. If that sort of delta plays a significant role in your overclock, you're courting disaster. Sufficient airflow through the enclosure should ensure that you're never dependent on a particularly high-performance thermal compound.
More hardcore overclockers tuning their systems competitively, or quiet computing enthusiasts who spin their fans down as low as possible for better acoustics might want to give more thought to their paste of choice. Even a majority of power users, however, should focus on a paste’s ease of use, leaning toward the softer compounds. A flawed application typically impacts thermal performance much more than the difference between pricey and value-oriented pastes ever could.
Graphics card cooling has its own set of rules. The optimized tightening of screws, the use of better thermal paste, and a more careful application of said paste (usually not possible during mass production) can all contribute to measurable gains. Furthermore, it is a good idea to include your card's backplate in the overall cooling solution, as this step is almost always worth the effort. In addition, it would be wise to question existing fan curves, and appropriately adjust them to any temperature changes. Just remember that modifying the fan curve makes the most sense after tuning your card's backplate, since you could see augmented cooling on components back there, such as memory modules.
Whether you're buying paste for your CPU or GPU, do plenty of research first. This is a comprehensive list, but new products come out all of the time.
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