Your PC's internal hard drive is the most critical component of your system after the CPU and memory. The hard drive is the place where your operating system, programs, and data are stored and accessed.
If you like movies, take lots of digital photos, play games and listen to music a big and fast internal Parallel or Serial ATA hard drive is the perfect choice. If you need more storage you can add an external hard drive which are available in USB 2.0, FireWire 400 or 800,etc.
Here are some articles about Hard-drives and how to choose them:
Shop like a pro for an internal or external drive.
Key Features
Hard-Drive Shopping Tips