Creative does claim that both the Audigy and the Extigy are capable of 24 Bit playback, however I have not found any software that will playback at that bit depth on these products.
However unless you are doing live mixing, 24 Bit playback will not significantly enhance sound unless your source is recorded at 24 bit depth, somthing neither the Audigy, or Extigy is capable of doing. Both cards are still locked at 16Bit/48KHz for recording. To record in 24 Bit you would need a Pro/Near Pro card such as the M-Audio Audiophile 2496, Echo Mia, or Terratec DMX 6Fire.
Unfortunatly, with the exception of the 6Fire, most (near)Pro cards are not very good at games and are generaly stereo only. Since the 6Fire was not available when I put together my system, I had to buy both an Audiophile 2496 and the Audigy for playing games, MP3s, DVDs etc.
AS for any "Enhanced MP3 encoding", I would think that that is a function of the software encodeing, not general consumer based hardware. However, I could be wrong; I don't ever encode MP3s (too lossy). I instead encode all my music (and recorded vinyl) to APE format using MAC(Monkey's Audio Compresser).