Well tipical line levels are +/- 1V but this is not standard, some sound cards output +/- 2V when put to full volume.
Well you could do it without amplification but you lose ADC resolution that way. If its just for a little audio DSP fooling around it would be fine but if you want something quality you should get a proper external 16 bit audio ADC.
Oh and its easy to shift the DC bias of the signal to half Vdd. All you do is put a resistor from the ADC pin to Vdd and a nother resistor from the pin to ground. It makes a voltage divider that makes 2,5V out of 5V. The resistors can be anythyng from 1K to 100K, Lower the resistor the lower the input inmpendance so then you also need a bigger capacitor to get a good response down to 20Hz