Looks like it's a regular square taper BB so finding an upgrade will be easy. A Shimano UN54 BB (around $25)would be a good, inexpensive option but there are plenty of other options out there. You'll need a crank puller as well as a tool to remove and intall the BB. Not sure what the 8 sided nut is. There should be either an allen or nut holding each crank arm in place and they should be 6 sided. If the problem is loose crank arms (as opposed to a worn BB), you can try tightening them up but they usually can't be saved if ridden very far when loose.