You should lift weights, but you should also get a trainer to help show you how to do it correctly and safely. Don't just rely on pictures in books or on the net -- if you screw up your back or knees, it can be permanent and debilitating.

That being said, lifting weights is great for you, doesn't take much time (you can do a very complete job in 4 hours per week), and has significantly helped me with back pain.