A good site will tell you what the wattage will cover in square feet. I think a 400 will cover 4ft by 2ft area.
Basically 40-50 watts per sq ft.
Get what you can afford. 600 if you can.
Don't skimp on the reflector. A good reflector makes all the differance. Well that's what I'm told by some of my friends that grow indoor plants year round.
Ryan have you gone to actual grow light sights and read about the advantages and disadvantages of different hoods and fixtures?
When I was considering getting a MH I searched and read and searched more to learn all I could about these lights, hoods, and ballast.
My first question is how do you intend to use these lights?
Are you going to do away with your enclosures?
Do you plan on using these lights over your enclosures or build another enclosure to house light and plants?
These are all very important details which you have not covered.
First you know the lights put out large amounts of heat. To help with this and especially if you plan on building an enclosure to hold light and plant the hoods that are enclosed with the round vents on the ends are used for such applications and a vent fan will need to be used to pull the heat through the vent pipe removing most of the heat from the fixture. There are several types of these hoods and some are much better then others.
Now if you plan on using the light over an enclosure you will have to vent the enclosure which kinda defeats the purpose of the enclosure. But you need to vent it with a vent on each side and normally a fan to pull cool air through. You can even wire the fan to operate off a 120 volt thermostats and have it come on when the enclosure temperature reaches a high temperature which will be often.
See not only does the light itself get hot but it has radiant heat as well and will cause a greenhouse affect on your enclosure probably raising temperatures into the 100s.
To give you an idea. Two 250 watt MH over my reef tank raised the water temperature in my 125 gallon tank into the high 90s killing everything in it. I'm not going to get into chillers and such for a reef tank. But this happened in less then a day with an open canopy. Meaning the lights were not sealed to the tank. So the temperature spike when my chiller failed was due to radiant heat.
This is a big reason why I got rid of the halides on my tank, and a major deciding factor with my plants as I like and they LOVE my enclosure. So until I get my LEDs made I will stick with fluorescent lighting.
The enclosed hoods are designed to get the heat away from the plants, not necessarily the light fixture though it does prolong bulb life.
You have only two trees ? Get a 250 watt !!!!
I have been growing indoor bonsai under metal halide lighting for years. While high output fluorescents can be used, bonsai tend to remain rather static under such light. (That said, I find T5's very useful for propagating cuttings and very small trees.) Under halides, my indoor trees display significant growth and thickening. Yes, they can be expensive to run, and yes they run hot, but the rewards are well worth it. My best advise is to get yourself a good digital/electronic ballast, NOT one of the old coil and magnet kinds. Besides running bulbs much brighter than the old magnetic type, digital ballasts can be used to run different wattage bulbs. The new air cooled lumatek, for instance, will run a 1000 watt, 600 watt or a 400watt metal halide. This means that you can start with a 400 watt bulb, and if you love the results and want to expand your collection and set-up, you can simply pick up a 600 watt bulb, and set the ballast to 600watt.
My only caveat is that you should not dim HID bulbs. While all the new digital ballasts claim they can be used for dimming, I disagree with this practice. If you have a 600 watt bulb, run it at the 600watt setting. HID bulbs were designed to produce a particular quality of light at a particular wattage, and you have no idea what you are doing to the quality of light by dimming it.
Ryan,
Here's a link to a US store that has them at a pretty fair price. I just bought two of them, and they arrived in perfect condition. Remember to order the proper lamp cord with the ballast. These ones in particular use the "lock and seal" connection. Just search around the website for it. And like I said, start with a 400 or 600 watt bulb in the 6000 - 7000 kelvin range. Then, if you like the results, you can go up a size without having to swap ballasts. Have fun, these really are sexy-looking pieces of engineering!
http://www.hydrogalaxy.com/growing-lights/ballasts/lumatek-air-cooled-dial-a-watt-dimmable/
Hard to believe that a grow store doesn't have a variety of halide lamps. They should have everything you need and then some, including digital ballasts. What kind of a hydro store is that ?