You can pick up 1TB or even 2TB external USB hard drive that will easily be powered by the RPI 3B and will hold pretty much every system you can think of on 1 TB drive and have room to build a media centre
If you use a 2TB external USB powered drive then you'll also have plenty of room to put movies, etc, etc as well as plenty of games to play
If you choose to get an external hard drive , make sure its a decent brand and a true 2 1/2 hard drive (I have 2 1TB Toshiba USB powered and they are tiny) . They are smaller than the RPI in a case as I'm aslso looking at building a media centre with games that will fit on the back of our LED TV using the VESA mounts once I finally finish off the arcade desktop cabinet I've been working on since Christmas !!
https://www.modmypi.com/raspberry-pi/cases/vesa-mounts
I have to order the buttons and sticks and its pretty much done, with wi-fi if needed, but is really designed to plug an ethernet cable into it and USB keyboard as needed for updating, plus PS3 joypad for PSX games also buttons on the side for pinball DOS games
I may upgrade to a full hyperspin desktop cabinet as I have a spare Intel i5 when I upgraded my desktop to an Intel i7 along with getting a cheap motherboard with on-board GFX and 4GB RAM, but thats another future upgrade well down the to-do list. Actually getting it finished before this Cristmas would be a start