Renting a room in a hotel? Need info.

Oh, let's say I'm going from ... Pennsylvania to Montreal, Quebec around the end of June 2009. When should I reserve my airplane ticket (with hotel!!)? Also, if I was to rent a room in a hotel, and my girlfriend wants to get in. Do I need to pay for a 2nd person? Or does she only need the key to my room? I always wondered about this, since I have only been to a hotel ONCE in the past (and that was in 7th grade.. which was 7 years ago..).

Also, I noticed prices for airplane tickets from here to there, including hotel costs are around 650 - 1000 (+) for about 4 days or so, seems cheap (at PriceLine.com <3)!