The "touch" portion of the touchscreen connects via USB or Serial, depending on the model. Think of the touch overlay as a big, flat mouse, and it connects to the PC as such. If you already have a LCD and want to add a touch panel to it, check here:
They offer touch screen kits for LCDs from 5.6" to 17". The kit consists of two parts, the touchscreen overly and the controller, which is USB.
As far as USB vs. Serial goes, I'm not sure it really matters a whole lot. Since manufacturers are producing a lot less serial devices (if at all), it really boils down to supply. I haven't read any posts here about glitches in serial touch screens...but that doesn't mean there aren't any.