Insulation, Radiant and Vapor Barriers - attic ventilation and insulation

We have an attic ventilation fan going out of the roof, but we don't have any other vents going into the attic. Should I put vents in the soffits? Also, is the insulation in the attic suppose to be all the way to the walls and tight in all the corners? We have fiberglass insulation on the floor inbetween the raftors, then styrofoam, and then another layer of insulation. Is that causing a vapor barrier?

http://www.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11810

This site provides some useful information about your questions. The Styrofoam between the layers of insulation violates the 5 to 1 rule noted on this site.

If your attic fan is rated at let's say 1500 cfm, then you must have attic vents rated at 1500 cfm. Having no vents will cause the fan to eventually fail.

An attic fan will pull air out of the attic to the exterior. In order to function, it must draw air from somewhere else.
If you had other vents in the roof, it would have drawn it from there, and it may not have created the convective current you need in your attic to remove heat calories and humidity.
If you have enough soffit ventilation and it is unobstructed by insulation or other debris, then the correct flow is occurring.
If the soffit ventilation is blocked, the fan will 'suck' air from any pinholes and openings in your house (plumbing [stack vents], ductwork, electrical outlets and electrical conduits drilled into top plates, etc). This will affect your home's 'thermal barrier', in turn your energy bills, and worse: 1. if you have a gas furnace or water heater - carbon monoxide will be drawn back into the house - plus 2. if you happen to have a radon gas problem emanating from the soil, the gas (second leading cause of lung cancer in the US) will be 'sucked' into the home as the home energy gets sucked up into the attic and out the roof.

Please consider conferring carefully with an Energy Efficiency Auditor (Analyst) and a Radon Gas inspector in your area before considering your building envelope.
Many states are offering help in this, and with the Energy Bill passed recently by the Federal Government, many state agencies are administering funds to be used in energy efficiency education and also the actual mitigation, especially if your income level allows you to qualify. (The government is offering tax credits on energy efficiency in the home up to $1500; for low income homes [150% of poverty], new furnaces, refrigerators and the cost of insulating and air sealing the house; among other further credits too: for upgrading to a more fuel efficient vehicle, btw).

A vapour barrier is a very good idea, if its installed in the right place for the area you are in.
If you are in a cold place, then the vapour barrier should be just under the inner surface of the walls and ceilings.
If you are in a hot and humid place then the vapour barrier should be just under the outer surface. Presuming the inside of the home to be cooler.
Having a vapour barrier such as polystyrene above the attic floor, will in time lead to wood rot, if you use heating during the winter.
The purpose of a vapour barrier is to stop the water vapour created inside the home rising into the roof space, or into the walls, forming condensation and mould on the timbers and rotting them, and possibly making members of your family ill.