High Resolution ADC for Noisy Sensors in Variable Conditions
ADS1256 from TI has eight single-ended 24bit channels with high-impedance input buffer and PGA. OpenEXG project has PIC code to interface it (they use two channel version ADS1255, but it should be the same).

If you want differential inputs, then there is ADS1298, with 8 channels, PGAs and A/Ds, internal reference, plus ECG/EEG circuitry which you can ignore. I am not sure you can find any example code for this one, though.

If you are looking for resolution, then precise, low noise reference is a must.