adding intern harddrive

Hello fellow dell/allienware users.

Now I'm  not a big expert on laptops/computers and that sort of stuff, but..I do know a lil bit about it to get around I guess.

I bought an MX17 Alienware laptop about..a year ago, and decided to only have like 120 gig of harddrive space, this was a massive mistake, because I do play quite a bit of games, and it was full in no time, and now I keep having to deinstal everything before installing a new game, which is quite trouble some, I even had to put all my photos on a portable harddrive, which is even more anoying. now a month back I decded to clean my laptop on the inside (and boy was it needed, my entre van was clogged up) and I found a free open space in my laptop, which was called HDD1. now I talked to a friend of mine, and he told me I could put a harddrive in there, but what brand and what I should look for, he didnt know. so now I come here, What should I look for if I was to buy a harddrive for my laptop? are there brands that only dell only works with? or can I just fit in any laptop harddrive?

thanks in advance.