ATX, or Advanced Technology Extended, and Micro ATX are the standard form factors for computer motherboards. The most evident difference is in size. Micro ATX motherboards are considerably smaller than regular ATX motherboards. Choosing one or the other can affect your future buying options--most notably, case compatibility.
Case Options
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ATX motherboards have more options when it comes to larger cases. Having a larger case opens the possibility of more advance options such as SLI or tri-SLI. Micro ATX motherboards don't have room for multiple video cards, which are ideal for the hard-core PC gamer, the advance computer user who needs a lot of computing power, or someone who wants to run multiple computer monitors at once.
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More Ports
Full-size ATX motherboards have the room to include more ports--including USB, PCI, and SATA slots. More USB ports means easier access to peripherals (e.g., webcams and game controllers). While not used very frequently, additional PCI slots can be used for firewire or sound cards. Finally, more SATA connections allow the possibility of more internal hard drives or disc drives.
Installation and Overclocking
Switching out and installing components is easier on an ATX board. There is more room in between components, and the ports on the board are generally clearly labeled. Additionally, full-size ATX boards can support more power, making them ideal for users who like to overclock their video cards or processors.