If you want to be comfortable, figure on a tent with 50% more capacity than you need. Tent capacities are based on an average sized Oompa Loompa or 6 year old child, or for people who don't mind being very, very intimate. For 3 people and a large dog, a 4 person tent will work, but won't be very comfortable, I'd go with at least a 6 man or better. If you plan on keeping your gear inside with you, backpacks and such take up a lot of space inside a tent. Also, a tent with a large vestuble (covered area that not inside the tent) is nice. That allows you to keep your stuff outside, without crowding up the inside of your tent.
Also, x2 on the square base, or something close to it. Hexagons and octagons just aren't very space efficient, there's a lot of wasted corner space.
Costco usually has some decent tents for reasonable prices. Sportsman Guide, Cabelas, Campmor, and a lot of the other mail order catalouges always have a pretty good selection at reasonable prices. REI also has good tents, but are geared torwards the more experienced outdoorsman, so they tend to be more expensive. They also rent equipment, that way you can try out different models before actually buying.
The larger family tents aren't really designed for hard use, but it sounds like you dont' really need it for that. They'll be fine in spring summer weather, but in a heavy downpour or strong winds they might not hold up very well.