1. You are talking about
bit-depth, 16 vs 24 bits. Sample rate is measured in kHz, and describes how fast you sample. Bit-depth describes how many bits are used for each sample.
2. According to the specs (
http://www.m-audio.com/products/en_us/FastTrackPro.html) to get 24 bits you have to configure the driver for "2x4" or "2-in OR 2-out" (whatever that means). If you have it configured for "4x4" you will only get 16 bit samples. And if you are not using any specific driver (just running it as a "class-compliant" USB device) you will only get 16 bits.
Bottom line... install the native driver, configure it for 24 bits.