You've got the zoom part right, a 2x teleconvertor will double your lens length and a 1.4x will give you 1.4x the length. The thing to remember is that they also lose you some light so a 2x teleconvertor will drop two stops from your maximum aperture size and 1.4x is normally a stop less.
Remember that with a crop sensor you are already getting around 1.6x the lens length.
For example, a 300mm lens on a full frame sensor is a 300mm lens, put it on a crop sensor and you get a 480mm equilavent lens. Add a 1.4x teleconvertor and you get a whopping 672mm equivalent!
If that lens is an f/2.8 then you'll lose a stop so the max aperture available to you is now likely to be f/4. Maybe not too much of a problem if it's bright but if you're in low light and handheld it might limit things a little.
Of course many zooms are not a fixed f/stop throughout (for example I've got a Sigma 55-200mm f3.5-5.6 which allows f/3.5 at it's widest setting but only f/5.6 when zoomed) so if you use a teleconvertor on one of these and zoom right in your f/5.6 will turn into something like f/8 which is really going to start restricting you unless you're on a tripod.
A 2x teleconvertor loses 2 stops so you're going to be at about f/11 which is just too slow for very much - this is why you see a lot of people using a 1.4x teleconvertor and why they recommend that you only use them on good quality (low f/stop) glass.
The posts above are talking about the auto-focus because most systems will only allow auto-focus when there's enough available light - close down that aperture with a teleconvertor and you risk taking it out of the system's abilities and forcing you to maunal focus.
As well as limiting your shutter speed the small apertures will also affect what depth of field you can achieve (as in, it will be very wide, without the nice blury backgrounds you sometimes see) but depending on your goals this might not be a problem.
That all sounded a bit negative didn't it?! You do gain a big increase in lens length but there are these considerations to be aware of. Have fun with your long lens!
