The mega is certainly capable of doing what you want. I have built a VGA board using the picaso chip from Sparkfun that drives a 7" LCD monitor. I am using it for a tire monitoring system for my large RV. There is also a text to speech chip available but it is currently out of stock. As soon as it becomes available I am going to hook that up for speech output on the unit. The mega has a total of 4 serial ports and since the Picaso uVGA controller AND the speech chip both take serial, this works out very well.
I have a full arduino library for the picaso chip if you are interested. It is very easy to hook up to the arduino.
With the mega, you could do all this and take a GPS input and your other sensors.
What the Arduino cannot do is a full moving map GPS, it does not have enough speed or memory.
As far as other platforms, the ARM is a good choice for LOTS OF MEMORY and SPEED. It however is much more difficult to learn.
Hope this helps