I would expect that if you want to invest your bitcoins that you would have to change them to cash and then you can buy stocks with that cash. Or, if the return is worthwhile, invest it into more hardware and whatnot for making more bitcoins, but it sounds like the return isn't very good.

The banking part of bitcoins sounds interesting but I am new to this so I can only speculate, sorry if any of this sounds really dumb but I am learning about this and its going to help to think this through...

It looks like there is no rule of law so perhaps its going to be very difficult to do something like loans. The bank right now shouldn't be much more than accepting deposits and charging a security fee to the depositor. People are scared anyway of something happening to their computer, like a virus or whatever, so perhaps there is a way to alleviate that fear. Good old fashioned bitcoin safe deposit box.