snip
Also, as other posters have said, the dimmers themselves normally have
to be rated at a higher wattage when used with halogen lights rather
than incandescent bulbs.

Sorry to be picky, but halogen bulbs *are* incandescent. "Ordinary"
light bulbs (i.e. with a filament in inert gas, but not halogen, etc)
are usually referred to GLS or General Lighting Service lamps.

This is because halogen lamps take a higher
current when first switched on. Typically 250 Watts of halogen lights
will need a 400W dimmer.

It is true that halogen lamps take a higher current when switched on,
but the use of a dimmer masks this inrush for low-wattage lamps. The
very act of placing a dimmer between the supply and the lamp creates a
soft-start effect which has the advantage of prolonging the life of the
lamp. The primary reason for de-rating the dimmer is to allow for
inrush/inefficiency in the transformer(s) not the lamps, per se.

HTH