You can't really tell the difference between 16bit and 24bit to be honest...24bit is just lower noise level. PC audio has plenty of problems with noise that a 16bit to 24bit differential is nonexistent...
24bit sampling isn't even physically possible right now anyway (144db SNR), the best DACs out there go up to 22bit (132db SNR). The X-Fi extreme audio has a 17 bit sampling rate (106db SNR). The best X-Fi, the Prelude, has a 20bit DAC (120db SNR).
The X-FI Extreme Audio is not a true X-Fi, it's actually just an Audigy 2 with different drivers, it doesn't use the same chipset as the rest of the X-Fi line which have > 18bit DACs.
You can have your PC stream 16bit or 24bit samples...you are still really limited to your DAC so the difference is not even close to 50% noise level reduction--the 132db SNR DAC i was referring to only has 38% noise level reduction.