It really depends what you're looking for. Just having a second HDD will not allow you to have different programs installed, etc... for each drive. You simply have your operating system on one drive, and you can put your movies on that drive and your work on the other (or something similar).
That'll work fine, but if that's all your after then a single hard drive with or without a partition may be more convenient and cheaper. If all you're looking for is more space, then that's probably the best idea.
If, on the other hand, you want completely different systems for work and movies (i.e. different programs installed, files that can't be accessed from the other drive, etc...), the hard drive enclosures I suggested earlier may be more convenient. You don't need to change cables or anything like that, you just turn off the computer, slide out one drive (you don't need to open the computer, it's more like removing a CD), slide in the new one and turn the computer on.
Aside from keeping independant systems, many people prefer this for security. For example, if one drive is used solely for work, it cannot be corrupted if you get a virus while surfing the internet on the other hard drive.