I just got a 24-bit card, but I'm sticking with 16-bit for a few more weeks. 8 extra bits per sample adds up to 50% more drive space for the same length track, and I'm in serious need of a dedicated audio drive.
I haven't compared 16 vs 24 on my system, but I'm a computer weenie so I understand the basic theory behind it. The bottom line is 24 bit audio gives you 256 times the "headroom" of 16-bit audio (from the 8 extra bits to store the intensity of each sample). Dynamic instruments can be tricky to record in digital, I'll take all the headroom I can get while tracking. I'm recording a lot of dynamic instruments, so 24-bit audio will probably justify the extra storage space, in my case.
96k, on the other hand, more than DOUBLES the storage space (over 16/44.1) and only gives you a smidge over twice the sampling rate of 44.1. For me, I think I'd have a lot of pieces in the signal path to improve/replace before my system would really reap the benefits of 96k.