The best piece of advice I can give you, with the lenses you have is.....don't bother with a tc one either of them, the 18-55 is too slow and too short for one to work, and the 55-200 is just too slow. Save your money (or sell your 55-200) and invest in a 70-300 VR instead
Just to re-iterate what Pete said; you'll lose 1 stop at 1.4x and 2 stops for a 2x TC. Also, depending on your model compatibility (Nikon will have a chart somewhere) you may keep your AF ability at 1.4x but almost certainly will only have MF when using a 2x.
Hope that helps.
The 55-200 is at 5.6 already at the long end, adding a tc is total waste of time and money