I've just fitted 100/90 bulbs to my bike - that's made a difference, though not as much as you'd expect.
More expensive than more powerful 'ordinary' bulbs, but probably the best you can get is an HID conversion kit, but you're looking at about £60 for the lower lamps and the same again for full beam.
Personally, I don't think HID is cost effective unless you have a vehicle with really crap lights, and the Multi's not that bad.