I think it's also worth mentioning that there is different techniques used to build a/d converters. Normally the tradeoff is precision vs speed. If you want resolution, it's going to be slow, if you want speed, expect low resolution. There are exceptions of course - precision + high speed = big dollars!

Your usual 8,10 and 12 bit adcs are normally 'successive approximation'. These are reasonably fast. If you want 16bits and greater these tend to be 'integrating' adcs. Most multimeters use 'dual slope' and the ADuC834 device I use is a delta-sigma (sigma-delta??) converter. Even though the ADuC834 is specced at 24bits, I only get about 16bits of resolution due to noise. At low voltages there's also temperature affects to think of - take your average diode (in4148) it's leakage is very low ,but leakage depends on temperature and can become significant to give errors if it is in your measurement path. There's also thermocouple effects due to dissimilar metals.

Analog Devices have a wealth of information on a/d converters and analog issues in general - it's worth a browse as it may save you a lot of time later on.