I read in gr0wer's "grow box design" thread that if you use more that 250W you should wire some copper, gfi outlets, and it's very own breaker. Is this a serious risk? I live in an apartment, recently renovated. I was thinking of a 600W or 1000W, but I'll settle for a 400W if I have to. This is going in a 6' x 3.5' x 8' closet grow.
depends on what is pulling on the circuit you plan on using, a 1000 hid would require a 15 amp by its self and maybe a few accessories.
worry less about the wattage and more about the amperage. Ideally, you should try and isolate your grow to a dedicated circuit. Does your apartment have its own fuse box? if so, try switching different breakers on and off, choose one breaker, and unplug EVERYTHING. figure out another way to power the devices you unplugged, and get your 15amp grow on a 20amp breaker. Should be more than safe.