Carlos,
Just go to
www.macsales.com, best source for anything Apple and all super reliable. Their prices are pretty good as well (although not as aggressive as some sporadic on-sale deals you may find elsewhere). You can easily see what hdd and memory fits your particular model Mac. I just upgraded an older MacBook G, both memory and hdd. Their support people will easily guide you if you have questions.
I am a very recent Mac user, or rather developer, since last month actually. And all I can say is that the first app I installed on the Mac was VLC player (free), simply hate QuickTime and iTunes and their proprietary file BS that most of real world has no use for. No need to convert files, whether music or movies, simply store them on the external hdd and be done with it.
Depending on which Mac laptop you have, replacing an internal hdd may be an issue, or not. Mine was a royal pain as you tend to pay the price for Mac's sleek profile and removing 18 screws or so plus unlatching the keyboard/trackpad/topside was as stupid an effort as it gets (and I can take my HP laptop apart in seconds flat). Bottom lie: google your particular model's hdd/memory upgrade instructions first, if, after reading the instructions, you still feel comfortable doing the job yourself, then go for it.
If you end up buying an external hdd for backup, just turn on Time Machine, it will automatically suck in all the files on your Mac and keep them updated as you go. I created a separate folder on my external hdd to hold music and movies and manually move them over, this way you only have one copy of them and keep your internal hdd uncluttered, but that is up to you, of course, if you want to keep multiples. And yes, this way you can preserve your original file without getting iTunes/Quicktime involved.