One poster rates using bulbs more powerful than standard (wattage wise). NOT A GOOD IDEA. This is because thanks to skinflint accountants the thickness of the conductors (copper) in the wires, has been whittled down to the bare minimum. A 12 volt 60 watt bulb takes about 5 amps so they don't rate the wire for 10 amps. Other problems are the bulb connector contacts, (often just tinned steel nowadays) these can burn-up a treat as can the lighting stalk. Some headlights have plastic reflectors plated with metal and these don't even like the heat of a normal bulb, the aluminising corrodes away near the bulb giving very dim lights (some Citroens were like this). Even if you have a plated steel (they don't use copper anymore) reflector, the extra heat going through the glass makes mud stick and bake on instead of being washed off by the rain.
Think headlamp relays if you do go this way but a lot of cars now have 4 bulb mainbeams.