What is the voltage limits for ADC

What is the max voltage that the ADC can take?
I'm pretty sure it is supposed to go to 5 V unless you change AREF

Can AREF be set as high as 9V++?

No, it's a +5vdc chip, and the ADC is using the Avcc pin for it's reference. There are ways to lower the reference to less then the +5vdc reference, but no way to raise it above +5 volts.

If you have analog signals that are higher then +5vdc or that have a negative voltage component then you need to utilize external components and circuitry to convert the signal to a 0 to +5vdc signal for the processor to utilize.

Lefty

If you need to measure voltages higher than Vcc, use a voltage divider and scale the result appropriately.
The applied voltage on an analogue pin can be higher than Vref (your readings will simply max-out at 1023), but it can't be higher than Vcc.