It is really a question of what you want.
You can of course use the external drive, not only as a backup but also as another HARD DRIVE on the computer.
It will be seen in My Computer and you can access data on it just as you do on your three drives , BUT of course ONE with three partitions - now.
How I do it and there are MANY ways is to simply copy my documents, images - ie: family phots etc, and my music files ie mp3 etc to the external drive.
I also have three backups on there - a Seagate from one computer, an Acronis from another and a Vista backup from another.
That said I use the computer not only for tbis but also for family research - a small part-time hobby.
That is a 1Tb - 1000 Gb`s and I also have a 250Gb external.
So in a nutshell an external provides not only a backup, that is separate from the hard drive in the computer but also additional storage.
If you are going to buy one I would recommend a powered one, rather than relying simply on USB power - not everyone would agree with that.
They are so relatively cheap that I would go for the largest capacity you can ever see yourself needing rather than the smallest.
PERSONALLY I only use Western Digital or Seagate.
Many makes of external drive - under the marketed name do not actually reveal WHAT the actual drive is.
I provide this link that follows ONLY to give you some ideas. It does not indicate any support from or to Geeks to Go.
Western Digital WDBAAF5000EBK My Book Essential External Hard Drive - 500GB, USB 2.0
Item #: W10-3124
This is an elegant, high-capacity storage solution for all the chapters of your digital life, now featuring a new visual backup solution.