CD's are all 16 bit. Anything you record in 24 bit will eventually have to be rendered to 16 bit if you want it to go on a CD. The earlier post about increasing the file size is spot-on. If you're doing a lot of work in 24 bit, you'll start bumping up against limits in CPU usage, etc. since the files are so much larger. The biggest benefits to working in 24 bit are if you've got high end recording equipment (with a DAW which will support all that expensive gear) and are doing professional work. Otherwise, you're not likely to hear much difference between 16 and 24 on most home recording gear. If you decide to buy the M-Audio card, buy it for the sound and don't worry about 24 bit.
As an aside, I have an M-Audio AP 2496 card and love the sound but hate the drivers and the lack of support from M-Audio, so caveat emptor.