Can humans perceive sounds above 20 kHz?
Yes, we can. By means of bone conduction we can hear up to 50 kHz, and values up to 150 kHz have been reported in the young (Pumphrey, 1950). However, it is indeed generally agreed that 20 kHz is the upper acoustical hearing limit through air conduction. The reason for this is debated, but the transfer function of the ossicle chain in the middle ear is a suspected culprit in setting the upper frequency limit to 20 kHz (Hemila et al., 1995). 

Hence, using normal speakers or headphones 20 kHz is a very reasonable absolute upper limit. Note that the Nyquist criterium necessitates higher sampling rates (at least 40 kHz), so your statement of using 50k-60k sound cards is correct. If you decide on using bone conduction aids, you might start to think on using higher sampling rates still.

Here is an example of a commercially available bone conduction head set (AfterShockz):



These devices have the potential to increase the upper limit because they bypass the middle ear and hence circumvent the limiting transfer function of the middle ear. They induce vibrations onto the temporal bone, that travel via the bone directly to the inner ear. See the following picture from The High Tech Society:



As a side note: when you grow older the hearing sensitivities at high frequencies  are severely reduced, and even the 6 kHz range is severely affected in the elderly (picture from John Perr's website):



Disclaimer: I haven't looked into the capabilities in terms of upper frequency limits of bone conduction head sets. I am just talking theoretical limits. 

References
- Pumphrey, Nature 1950; 166:571
- Hemila et al., Hear Res 1995; 85:31-44