The halogens used in overhead projectors are far from an ideal light source - they have terrible color rendering, are only slightly more efficient than incandescents, and are very low life - $10 for 50 hours of life is not practical for most people.
Metal Halides are the ideal light source - great color temperature / rendering index, around four times as many lumens per watt as halogens, and you can buy 20,000 hour bulbs at $40/bulb, though you must buy a ballast for $40-$100.
The halides have a negligible amount of heat reduction over halogens, but the fact that you get so much more light out of them is important, and allows you to use lower wattage bulbs.
