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...From what I can tell, the 430's are smaller than the 580's and don't have as much power. The II's can be controlled by the camera menus and the original 430 and 580 can't. That's about all I know.
What camera do you have?
...Is there any reason why I should go with the 430ex II over the 580? I've always read that you should buy the most powerful flash you can afford. I'm just not sure if the improved features on the 430ex II are worth giving up the extra power of the 580ex.
The extra power of the 580 over the 430 isn't a big deal in most cases, but is a factor. The bigger advantage is that it can act as a Master to control Slave Speedlites, which could be important if you're planning on getting into multi-light setups in the near future.
I prefer the original 580EX to the 580EX II because it has a seperate Master/Slave/Off wireless switch, compared to needing to go to the menu on the 580EX II.
If you'll only be using one flash, or if your camera's pop-up flash is Master capable, then the 430EX II might be preferable for its ability to be controlled with the camera menu (if your camera supports that), and you could get a 580 later as funds become available.
In any event, to learn more about flash I recommend Syl Arena's The Speedliter's Handbook and Neil van Niekerk's On-camera Flash Techniques as essential references on the what and how of flash.