I'm currently using a CS5532-BS to measure noise from dry EEG sensors. After some googlin' this afternoon and browsing through datasheets, it dawned on me that there isn't really a straight-forward way of comparing ADCs. It would be nice to have a website where one enters in the frequency range of interest and the voltage range, and then is given a plot of noise spectrum for various ADCs. You could also have a setup where you can add a number of different front-end amplifiers and see the final result. Alternative vendors include Linear Technology and TI. I've seen the LTC240x and the ADS121x mentioned on Google Groups though I'm not sure how up to date that is. The key, of course, is simply to have noise that is below that of your front-end. Having noise much much smaller really doesn't make that much of a difference.