How many ohm is the ADC port internal resistance of the atmega8?
Page 310 of the datasheet gives you your answer:



The analogue ports have an input resistance of \$100\space\mathrm{M}\Omega\$.



However, that is not the whole story. As with all ADCs, a sample and hold capacitor is used as part of the ADC in order to hold the sample value during conversion. This circuitry has its own characteristics. From Page 244 the following diagram is taken:



Here we see a 14pF sample/hold capacitor with series resistor to half the Vcc rail. As a result of this circuitry, you must use a low impedance source to feed the ADC.


  The ADC is optimized for analog signals with an output impedance of approximately 10k or less.


A \$10\space\mathrm{k}\Omega\$ or less source resistance is recommended, otherwise the low pass filter effect of the capacitor with the source resistance becomes a major issue, requiring a longer sampling time for conversion and as a result limiting the maximum frequency. A unity gain buffer can be used if the source is high impedance in order to negate the effect of the sample capacitor.