Using too much or too little thermal paste could lead to heating woes down the line. Plop a roughly pea-size dot of paste in the center of the CPU for best results. (PCWorld's guide to installing a CPU cooler provides tips for the entire process.) And don't forget to peel the plastic off the CPU heatsink in aftermarket coolers!
Speaking of which, if you're using an aftermarket cooler with a tower-style heatsink design, be sure to position the cooler's fans so that they point at the exhaust fans at the rear or top of the case. You don't want all that hot air blowing around inside the case—or worse, blowing directly at your pricey graphics cards.
Bringing it all together
With the previous steps done, it's time to place the motherboard in the case. Well, almost. (Your motherboard's I/O shield is in, right?)
Your case came with brass risers (or "standoffs") designed to keep the metallic side of the case away from the motherboard. Forgetting to insert those and flipping on the power could fry your PC's components, so don't forget them. Screw in the risers until they're finger-tight, but don't make the mistake of overtightening them, because brass threads are oh-so-easy to strip. And once again, read the manuals to ensure that you install only the risers you need. The risers you put in the case have to match up with the mounting holes on the motherboard.
Unless you're working in an ultrasmall case and have a compelling reason to do otherwise, install the power supply before you slap in the motherboard. Space can get cramped in a case once the motherboard is in place.
From here on out, everything is pretty simple.
The only major mistake you might make at this point is installing the graphics card in the wrong PCI Express slot. While many motherboards sport several physical PCI-E x16 slots, some of those may offer only PCI-E x8 capabilities. Always install your graphics card in the slot closest to the processor to ensure the best performance, and consult your motherboard's manual to puzzle out the best use of other PCI-E slots if you're installing multiple video cards.
Beyond that, the most common mistake is failing to think about cable management while you wire everything up. A clean case not only looks gorgeous but also has better airflow—a crucial aspect in keeping your PC nice and chilled. Think about how you're going to route your SATA cords, power cords, and other connections as you install them. Try to tuck as many as you can behind the motherboard tray, and remember: Twist ties are your friends!
The final mistake—forgetting to plug something in—trips everyone up at least once. All it takes is a single poorly seated connector to prevent your computer from booting or otherwise introducing pesky ghosts into your machine.
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