Before adding lights, I would suggest making sure that the existing headlights are operating at maximum/proper efficiency. Check that the lenses are clean, and check that the voltage at the bulbs is correct. If it's on the lowish side, putting in a nice heavy, direct connection from the battery (controlled by a relay at the bulbs) will help a good bit. This is a common modification to make on many vehicles (and is entirely legal and safe).
You can also get headlight bulbs that are brighter at the expense of lower bulb life. If you don't do much driving at night, that may be a very good tradeoff...or even if you do.
Supplementing headlights such that they're actually more effective at highway speeds is trickier than it seems at first. Often there's a tendency to illuminate the foreground more brightly, which looks nice and reassuring, but doesn't help with seeing things far enough ahead to react to them (and can indeed have the opposite effect sometimes.) Lighting appropriate for tooling around campsites or up back forest service roads at slow speeds is quite a different matter, of course.