ADC internal clock speed?

HI...

From ATMega168 datasheet (page 248):
"By default, the successive approximation circuitry requires an input clock frequency between 50
kHz and 200 kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the
input clock frequency to the ADC can be higher than 200 kHz to get a higher sample rate.
The ADC module contains a prescaler, which generates an acceptable ADC clock frequency
from any CPU frequency above 100 kHz. The prescaling is set by the ADPS bits in ADCSRA.
The prescaler starts counting from the moment the ADC is switched on by setting the ADEN bit
in ADCSRA. The prescaler keeps running for as long as the ADEN bit is set, and is continuously
reset when ADEN is low."

What is the default conversion clock on the arduino dev. kit?

it looks like it is set for 125 KHz.

Have a look in hardware\cores\arduino\wiring.c if you are inclined to play around with this value.