A key question is what's your primary use of this computer, and how much storage do you really need.
200 gig sounds like a laptop hard drive, so apart from upgrading to new equipment, your choices may be limited to good external storage.
Keep your applications and working programs on your internal drive, move all music, photos, videos, etc to the external drive.
You can buy externals very cheaply nowadays. I have a 500-gig Toshiba external drive that's hardly any bigger than my iPhone. Great for transferring projects to an offsite location. Cost me $79, functions solid state, like a memory stick, no power supply required, simple USB connection.
That might be all you need.
If you're on a desktop, the best boost is an eSata external drive offering a terabyte or 1.5 terabytes of storage. That drive will operate at the same speed as your internal drive, which is essential if you're working with video, but otherwise may not be necessary. It will also require an eSata card in your computer, which bumps up cost.
To answer your original question ... my computer has 4 internal hard drives, 1.5 terabytes each, with eSata connections to two external racks, same configuration, for a total of 18 terabytes. More than most people need, but for me it's a work machine.