Your external display, unless it is manufactured by Apple will require calibration. This is because the default settings on monitors are for every day use and for any kind of display card. While the internal displays on Apple products are calibrated specifically for the video card that is in that machine.
In regards to display types it all depends on what you want to do? Video/audio production, graphic design, gaming.
If its for gaming you'll need something that has a good refresh rate for motion graphics. Same with video, audio production doesn't really matter.
When it comes to graphic design you will need to ensure that your display can reproduce the color spectrums you are working with.
The cheaper the display the cheaper the technology in the LCD.
For really nice color reproduction and refresh rates you will be looking for a display that uses S-IPS/H-IPS LCD panel. For middle of the road you'd be looking at PVA/MVA technology which has ok color reproduction and ok refresh rates. Then there is TN panels which are the bargin screens.
You can read more about LCD technology over at Wikipedia.
I would recommend an IPS display from either Samsung, Dell or Apple.