The difference between 16 and 20 bit quantization is an expansion of dynamic range, as well as increased low-level detail resolution. 20 bit resolution actually represents the state-of-the-art with regards to digital audio reproduction. Try some XRCD material to experience 20 bit resolution on your Redbook player. Some DACs have 24 bit capacity, but the last 4 bits aren't precise enough to encode music accurately. Some advertisers claim 24 bit resolution as a marketing ploy. The short version: There is no sonic advantage between 20 and 24 bit(yet).
