think you mean 24bit.
that's mostly for having extra dynamic range for further processing. You do need a sound card capable of that if you're going to record vocals/external instruments.
Re: Sound cards - They do not affect the sound of the track itself, as the processing is done within the DAW, what they do is that they provide you with inputs (for recording) and outputs (for plugging your headphones/monitors in) that are usually higher quality than the inbuilt interface.
edit: try asking here if you need more info