Thanks to these boards I knew to go online Saturday and buy my tour (they sold out for the days I'm in Rome by Thursday), and because I'm not getting a Roma Pass I went ahead and purchased my admission too. I got my two emails shortly thereafter (one for the tour, the other for the payment).
Will I just show my email to the tour guide? What if I want to visit the Forum the day before, do I just show my email to the ticket taker? I CAN do that, right - visit the other two sights the day before and then the Colosseum the next day?
Thank you so much to everyone here, you have no idea how appreciative I am for all I've learned that will make my trip so much more enjoyable. What did people do before the internet?