Higher bit depths are important, too, as they give you more room in which to work without clipping. 24 bit is excellent value, and the difference is audible. Brooklyn is correct when he says that most people are listening in 16/44.1, but this is after mixing and mastering, at which point the audio is dithered to 16 bit, as CD audio only runs at 16 bit/44.1 KHz. Higher sample rates are useful in top line studios, as some top hardware converters work more efficiently at those rates, not because it necessarily sounds better.
In Cubase, all internal processing is done at 32 bit floating point, which gives infinite floor- and headroom, so clipping internally is not possible, as the effective range is in the thousands of decibels.
After 24 bit, the difference is not going to be audible.
Buffer sizes have nothing to do with Cubase, it's true, but they also have very little to do with your soundcard. They have much more to do with load on your CPU. Setting your buffers to a higher setting reduces load on your CPU, as more of the incoming/outgoing audio is buffered in RAM, rather than being directly processed at too high a rate for your CPU, which is why you get pops and crackles on lower buffer settings. If you are getting up to around 512 or 1024 sample buffer sizes and you are stil experiencing pops and clicks, it would suggest that your CPU is no fast enough and/or you need more RAM.
If you look at the Cubase fora at
www.cubase.net , you will find a huge amount of information on this as it pertains to your application.
