I beg to disagree in some aspects here. To begin with, desktops's prices decrease along the years just like laptops do, but it's easier to just show someone who doesn't really know about computers your mega/funny/hardcore case and he will fall in love without even a remote idea of what's inside. If a system lasts you two years, you're doing it wrong. 17'' is enough of a monitor, otherwise, your biggest tv with hdmi will do it. Is really a pc the way to go for intensive gaming? nope, the way for intensive gaming is an xbox one, a ps4 or a wii u while sitting in your couch watching your biggest tv playing every game available without having to do any modifications neither to your console nor its software and all at half the price of your "gaming desktop/laptop". I agree with the upgradability and the heating points though, It's better to lower some settings than to fry your laptop
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As I've said before, don't compare your watch and your smartphone. Desktops are for general usage in your house while laptops are for general (although lighter than pc's) usage in your house or anywhere as long as you can carry 1kg to 7kg.