This is correct, at least in the USA. I've noticed the electrical code is far for lenient in the UK.
Just keep in mind a 600w hps lamp, and I would imagine 600w of cfl draw a little more power than what they are rated at. But yeah, a 600w even with fans at what not should be ok on a 20amp circuit with no less than 12 gauge wire.
When running steady, a 600W appliance is pretty much equivalent to any other 600W load (inductive loads like motors cause a phase shift between the voltage and current and computers introduce noisy currents into the line voltage but this usually doesn't make a difference to the user and I just mentioned it in case anyone wants to quibble
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However, on startup, inductive loads (pretty much anything with windings eg motors and transformers) need to draw very large currents to get going. It takes energy to build up the magnetic fields in the winding and to get the rotor spinning if it's a motor. While this is happening, the appliance draws a much higher current than it's wattage rating implies. This is sometimes called the 'surge current'. You can see this when your fridge motor kicks in and the lights dim momentarily. Resistive loads, like incandescent light bulbs don't do this.
This has practical consequences for breaker size. An appliance that should run on a 15A breaker may trip the breaker on startup. I have a metal chopsaw that does this. It's rated 15A but it will trip a 20A breaker unless I spin the blade by hand before hitting the switch. I don't do this. I plug it into a 50A breaker.
The small magnetic ballasts in flourescents are not usually a problem, even if you have a lot of them because they dont start up at exactly the same time, but the big transformer in a magnetic HID might trip a breaker if the total load is too close to it's maximum.
