If you are going to Rome, I really can't recommend a visit to Ostia enough. It is every bit as good as Pompeii, and in some ways it's even better. Pompeii is, of course, awesome, but Ostia is very different. Pompeii was a wealthy city, while Ostia was much more working class, as it was a port town. Also, you can visit Ostia from Rome using a single 1 euro metro ticket (make sure to also have one for the trip back). It couldn't be easier.
Ostia was so important because the sea-going vessels were too large to travel up the Tiber River, so goods were loaded off the big ships and loaded onto barges for the trip up to Rome. Even after Ostia was replaced by the new ports built under the orders of Claudius and (I think) Trajan, for many years Ostia remained an important administrative center for coordinating the new ports.
If you want to read more about Ostia, please check out this website. It really contains an amazing amount of information http://www.ostia-antica.org/
Goods were stored in horrea, or warehouses, such as this one.In addition to mosaics, there are many frescoes extant at Ostia. This one was in a bar/pub/restaurant.