Second the motion on the 420EX. While the 550/580 has more oomph, there comes a point where portable oomph is sufficient. The 500 series' other advantage is that they can be masters to the 400's slave, but since like me you have a tight budget, it's not like you're going to be running out and buying a couple of 4XX or 5XX to be slave flashes anyway.
I took this picture:
Near twilight in a forest with a 420 for fill, using indirect and a diffuser cap! I have another picture I took *after* sunset (during twilight) from forty feet away with the 420, which looks like the subject is being illuminated by a brilliant full moon, including water ripples. I don't have it handy for posting at the moment, but you can see it here:
When I took that picture it was so dark I couldn't even see the model through the rangefinder. Exposure was around 1/30th and aperture couldn't have been bigger than f4, ISO 400. As I said, it was after sunset and the model was forty feet away. Unless you're going to have more challenging shooting environments than that
, I think the 420 will be plenty o' flash for you. [img]/ubbthreads/images/graemlins/smile.gif[/img]
Whatever you buy, just keep in mind that the ability to pivot the flash in both dimensions is crucial. If you offered me a 580EX that wouldn't pivot for my 420EX that does, I wouldn't even have to think about turning it down. The 580 does pivot, but I was just using that as an example.