Building a greenhouse

Im going to be building a greenhouse this weekend. Im going to still be growing my plants in pots, not in the ground. My question is should I leave the floor of the greenhouse the dirt ground or cover the ground with something? Im going to build a set-up like this one only smaller ( 8x5 )

http://web.archive.org/web/20060211143322/users2.ev1.net/~sivek/greenhouse/greenhouse.html

BARE GROUND, MY SUGGESTION WOOD B 2 CUT BOTTOM OF CONTAINERS OUT, THAT PLANTS R N AN SET THEM HALF WAY INTO HOLES U DUG INSIDE GREEN HOUSE ROOTS WILL EXTEND INTO GROUND, WHALA!! MONSTERS

I dont wont them growing into the ground. I am going to be using 3 gal pots. The greenhouse will be about 6 foot tall at the highest part. I want to be able to move my plants around as needed.

Pink -What size pot do you use? What should the floor be made of? Im a novice, have no clue of what to do.

we put gravel down on our floor.. and 3 gallons is very very small - 5 gallons is a minimum - closer to 35 gallons is what they really want...

Thanks for the info.