You'll be surprised on how much room a pot plant takes when in bloom. If you split the room, you should expect to be able to grow one plant on the bloom side at a time. Doing a SCROG will give you the best yield in a 2x3 space, but again with only one plant.
I'm using to stand-up cabinets for vegging that are approximately the same size (together) as your closet. Here's a pic:
https://www.rollitup.org/attachment.php?attachmentid=2683910&d=1370262823
This pic was taken at the beginning of the month, and the plant on the far left has now taken over the entire space and it's only on it's second week of flowering, and should almost double in size again before it's done.
The problem with HPS is heat. I've recently had to temporarily retire my HPS to finish a White Widow grow (check out my grow journal) due to it heating up my 4x6x7 grow room too much. HPS is the desired lighting for flowering (I HIGHLY recommend T5's for vegging...please refer to the pic above), but if you end up having to do a ton of gyrations to keep from cooking your plants, or cannot vent your closet outside (via carbon filter if you don't want the neighbors knowing what you're doing), going with T5's in flowering is a more sane solution. There's a ton of things you can do with T5's as you can get bulbs from aquarium shops that give amazing yields. We'll get into that more later.
Soil or hydro? It's a rule-of-thumb that anyone new to growing should start with soil first. Screw something up with hydro and bad things happen pretty quickly and you'll end up buying your weed from the guy on the corner again. Soil is more forgiving as mistakes (you'll make plenty) are easier to correct.
Back to splitting up your closet......I like having plants in various stages of growth. Considering that it takes about 4 months to go from seed to harvest, that's a long time between grows (I'm not a patient person). I'm still trying to get my timing down right, but if you start a new seed at the time you put your first plant in bloom, you'll have a harvest every eight weeks instead of 16....roughly 8 weeks of veg, eight weeks of bloom depending on strain.
If you want to grow multiple plants at the same time, you can probably put 3-4 in your closet and in four months harvest 'em. You will find the biggest thing you'll learn on your first grow.....is patience. The first grow fuckin' takes FOREVER! The plants I'm about to harvest were started Feb. 2nd.