Upgraded from 5200 RPM 500 GB stock to a 5200 RPM 2 TB drive that fits inside the case. Some Samsung one..... link:
http://www.amazon.com/Seagate-HN-M201RAD-Momentus-SpinPoint-ST2000LM003/dp/B00I8O6OQ4/ref=pd_ybh_1
Nothing fancy, great price, great amount of storage, and it does the job.
I'm expecting to last a while. With all my current games installed there's still 1.35 TB left... so more than enough room for many more games even if they all clock at 50 GB.
Glad I did it now, price was great and doing it later would just be more hassle. Had to install like 8 disc games... Bloodborne, TLOU, DriveClub, GTA V, Order 1886, Ground Zeroes, NBA 2K15, Killzone SF.... and just download a bunch of games again..... thank god for no bandwidth limit.
If you're like me and prefer to have all the games ready to go whenever you want, I definitely recommend upgrading, much less to worry about when a new game comes and try to figure out what to delete to make space lol. I ran out of HDD space after Bloodborne installed basically, was only like 13 GB left on the 500 GB drive.
I wouldn't worry about getting a much faster drive or anything tbh, this 5200 RPM classic HDD is working great, just aim for 2 TB or more storage and you should be set for a long time I think.