Hi VT, welcome to the forum. :thumbsup:

Both HPS and CFL will work, I personally would go for the HPS though, but thats down to personal preference. You may need to strenghten your grow box, but thats not a bad thing to do anyway. You will need reasonable ventilation whichever way you go as both type of light will put out a lot of heat (even in your cold climate) and you don't want a plywood based open fire in your house! I notice you say you have a small fan, is this for circulating air inside your box or for extracting stale air and drawing in fresh air through your box? You will want both for the best effect (fresh air = more CO2 = better growth).

6400k CFL's are great for vegging though.

A 400w HPS would be a nice size light for your area (8 square feet). Don't worry about cubic feet, it's the footprint that's important because you will have the ability to move the light up and down, thus rendering the third dimension obsolete.

If you can post a pic or two it would be helpful.. good luck :thumbsup: