The screen (cage) encircles a 30cm (12-inch) floor fan and is about 50cm (just under 2 feet) in diameter. All the plants sit outside (around) the cage - it is used for support and to train or weave the branches around the lamps to fill in any gaps.
The cage is normal fencing wire/trellis and is 120cm (4-foot) high. I usually grow 6-8 plants around it. Up to six hazes or sativas (because they branch out more) or up to 7-8 indicas (if they grow in a column). The more plants, the better the yield. I run both bulbs all the time - 1200w is 1200w, and the plants certainly like it! They grow much quicker with more light.
As for the overcrowding question, it's hard to say, because you need to know your strain: how much it branches, how much it stretches, and how tall it finishes. I like tall plants (up to around 150cm, or 5-feet), and I prefer sativas - especially hazes - as they grow long, sticky branches that are easier to weave around the cage. The lights will penetrate big, fat indicas, but they tend to have more brittle branches than sativas, and are harder to weave.
One trick I like to do is alternate my plants - sativa, indica, sativa, indca etc - as the indicas tend to stay shorter and get most of the benefit of the lower lamp, while the sativas can "fan out" at the top and fill the light gaps directly above the indicas - filling the whole screen. They benefit from the top lamp. You can see a bit of an example of this in the photos above, but it's not a good one as I had harvested most of the other plants when I took the photos and am just starting again. That last remaining haze will be harvested the day after tomorrow.
I generally hang the lower lamp one third up the plants and the top lamp two thirds up - the light overlaps in the middle. I raise the lamps as the plants grow.