You are going to need a good exhaust fan for a 400 watt HID lamp. My cabinet is 4ft tall with 250 watts of lights and a 120CFM fan just barely keeps temps in check. If you are constrained with height then you can always grow scrog or go lowryder (like I do), but the other posters are correct - if you have the space then go as tall as you can and you won't be limited by the height of your grow. Ventilation is the key to a cabinet grow - get a big exhaust fan. You can always dial it down if you need. Make sure you exhaust out the top and draw in air from the bottom. HOT AIR RISES. If sound is an issue then get a S&P Mixvent fan - they are quiet and move alot of air. I would avoid the Home Depot inline fans. I had the 6 inch which did well, but it was loud/whiney. Do it right the first time and you will save alot of cash.