To put it as simply as possible, as it seems you are quite new to photography.
No, your 18-200 will not behave like the 50mm.
The 18-200 is a variable focal length lense (means you can zoom in and out) with a more restrictive aperture window than the 50mm (lets less light in).
The 50mm is a fixed focal length (doesn't zoom), however it has a wider aperture, meaning that it allows more light into it (enabling you to use a faster shutter speed).
In addition to this, because of the 50mm lack of internal fixtures, it will more than likely create crisper and sharper images than the 18-200.
Lastly, because of the wider aperture on the 50mm, you can create a shallower depth of field at higher shutter speeds. As with your 18-200 you would need to be zoomed in quite a bit to get the same depth of field.
Anyways, the 50mm is extremely cheap, and for those that can fit that lens, it is a "must have" for any new, amateur or even professional photographer.