20 bit players vs. 24 bit players

"if most redbook cd's are only 16 bit??" - what do you mean by "most". All redbook CDs are 16 bit. Sony-Philips never created any other definition. Even HDCD is recorded in 16 bit format where bit 16 (least significant) switches dynamic range of music encoded in 15 remaining bits.

Extra bits come from digital filtering - pretty much useless information. Sonic benefit is, as Shardone explained already, improved filtering with possibility of reduced jitter. Improved more accurate filtering in digital filters gives smoother more coherent sound while reduced jitter lowers noise floor.

I would not put too much attention to number of bits or specifications in general.

Ehider - All 24-bit DAC are sigma-delta. Traditional architecture reaches only 18 maybe 20 bit. There are many reasons for that and one of them is immunity of sigma-delta converters to power supply noise. Sound of these converters is quite different. Quantization noise is higher but moved above audible range. SACD is pretty much the same as byproduct of sigma-delta before filtering and new studio DSD recorders are wide/mulitibit version of SACD. Texas Instruments released even DAC that is a combination of both technologies. Analog Device converted to Sigma-Delta. I absolutely agree that the sound is very different and not to everybody liking. Many people don't like sound oversampling converters. To them sound of traditional DAC is more organic with better reverberation. Sigma-delta is, in my opinion, more smooth and accurate.