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First: price range, tell us
Second: define "decent games". You'll probably need a laptop with a dedicated GPU to play modern 3d games.
for good price-for-power I'd suggest Asus. They're pretty good at hardware (but their recent trackpad drivers suck cock)
>switch hard drives
I HIGHLY recommend an SSD. It'll make a lot of shit faster, more than having a good CPU or a lot of RAM would.
But anyway there's usually a few panels on the underside of the laptop. At least one for RAM and one for drives. Unscrew and pop that off. The hard drive will probably be a in a metal tray (drives differ in thickness so they use a mounting tray to keep them all aligned). Undo the screws holding the tray in, slide the tray back so the drive disconnects from the laptop, and then unscrew the drive from the tray. Do the same thing in reverse for installing - screw drive into tray, slide drive into the connectors, screw in, put on panel. PS This pic shows two drive bays but most laptops only have one unless you get some nice shit.