How could I show CPU and memory information in the status bar?

as Michael said, with i3-status not, but you could use something different like py3status instead, see http://www.ultrabug.fr/meet-py3status/ (not enough karma for links) which is more flexible than i3-status

Even tho Michael is right below and you still want this kind of information anyway you can indeed use py3status to do so. In fact there's a contributed module that just does the job : https://github.com/ultrabug/py3status/blob/master/examples/sysdata.py. Hope this helps !