What cadence should I aim for?
For any given speed, you can either spin at a higher cadence in a lower gear, or a lower cadence in a higher gear.  The high cadence + low gear combination should reduce the strain on your joints since you don't have to push as hard.  You just have to do it more often.

I like to ride around 90rpm and sometimes drift up to 100-110 especially if I'm trying to catch up to or overtake someone.  I'll drop down to 80 for long, steep climbs (seated -- no idea what my cadence is standing).

Lance Armstrong apparently maintains 110rpm for efficiency.  It took me a while to get used to 90 so I'd suggest building up to it slowly.  Let us know when you can do 110 comfortably!