I've been 3 times, 2006, 2007, and last year. The group I go with has been going since 2003 I think, and will be there again. Bunch of Auburn University guys, the one that organizes everything is from Atlanta. The website, munich-bound.com hasn't been updated in a year or so, but you could get in touch with Mark on there and he'll see what he can do as far as tent reservations. What days are you going? We usually go the last week and are there for "closing". That being said, if it's just the two of you, you should be good to go. You should be able to find a spot at most tents, at least outside in their outdoor seating areas. Hofbrau is easiest for Americans, it's the biggest, but it's also more touristy. The "pigpen" in the middle of the tent, you don't have to have a seat to order a beer, but it does live up to its name. You have to have a seat to order a beer at all other places. I prefer the beer at Augustiner, but they're all good (although, I think the Paulaner is only passable when you have the other 5 excellent choices).
Be polite when asking about a seat, the outdoor areas are usually first come first serve. But don't be shy either, you'll lose your seat. Go early in the day, especially on Saturday. I'm jealous, I'll be back, but it'll be a while for me.