Rarely will you find any real-world AD converter with more than 20 bits of true resolution. Although does not advertize it, you can guess it from their published S/N ratio. And, well, not all of them are too careful when measuring things.
20 really good bits would give you an S/N of about 120dB. Several of the "so-called" 24 bit boxes spec at below 100dB S/N which to me suggest that their true resolution is about 16 bits (16 true bits is about 96dB S/N). The Edirol R1 I tried measuring on came in at about 15 bits (it might have been a faulty individual though) so the last 9 bits were simply meaningless noise.
In conclusion, if you have a quality 20bit converter you are about as good as typical converters get today. I would see no reason to upgrade that as probably the S/N is governed by the analog circuits rather than the AD.
Gunnar