You used to be able to purchase tickets to the tower online but it doesnt seem to be available anymore on the www.opapisa.it site.
Does anybody know if they have discontinued this or moved it to another site?

http://www.comune.pisa.it/turismo/torre-gb.htm

Are advanced tickets recommended for the Tower? We are going in August, so I am not sure how the line-ups will be....

The Opapisa site does work, I didn't see the link last night. I don't know whether the tickets are necessary in advance or not. It's just a convenience for me. I hate to stand in lines.

You can't just go to the tower and stand in line and walk up. You have to have a reserved time. If you go without a reservation, you go to the ticket office and reserve your time then. If it is a busy day, your reserved time might be hours hence. You are then left wondering how to fill 2-6 hours in Pisa. I'd advise to reserve in advance unless you think you might cancel later (you have to pre-pay.)