You might want to invest in a single Mono Flash such as an Alien Bee B400(160WS)
Paul C. Buff - AlienBees B400 . They are cheaper than your average PTTL speed flash and the quality of light is much better. When it comes to lighting, usually bigger is better. With a speed light you might be able to accommodate a few accessories like an umbrella or a simple soft box, but with a Strobe you can do much more, plus you only need one. You won't have to worry about battery life because the Strobe plugs into an outlet so you can fire away at will without having to worry about running out of juice. You might be asking suppose there are no AC outlets around. Well Alienbees can be connected to portable power packs that hold enough power for an entire shoot, shooting at full power.
Or you might want to go the Strobist route(speed lights) which is what many photographers are doing these days. The thing is, most photographers who switched to speed flashes already have their lighting down pat so they don't need things like modeling lights and light meters to get good shots. Using a Strobe is easier and you learn more about lighting in the process....