You should always be striving to upgrade your houses! Houses are what generate prestige, taxes, and later the very important Senate Points! It is a good idea to always have at least one housing upgrade in progress!

It is also important to remember to be upgrading your production buildings. Not only does this help produce more goods, but it also grants you the Urban Points that you will need to unlock additional suburbs and to move your suburb. Having very high level production buildings will also encourage your fellow citizens to come to your suburb and collect your goods, which is beneficial for both of you!