Cheap flashgun for Nikon DSLR?

I've been interested in photography for a good few years now, but I've recently decided I want to venture in to the world of flash and learn more about lighting…

I'm considering either the SB-600 and the SB-700, but I'm really stuck with what the differences are between them both… Could someone give me a round up of both with pros and cons, because as a newbie at more advanced lighting, I'm really confused, haha.

I'm also considering the Nissin Di866, but I'm stuck as to how I'd use that off camera with my D5000 (because atleast with the SB600/700 I can use something like the SC-28 cord to trigger the flashes…

I'd really appreciate someone to just clear this up for me and give me some advice.

When I researched this for my Nikon D90, I chose the Digital Concepts 918AF Flash. It rotated in both directions, had a diffuser and met my budget. It works just like I wanted.
http://www.amazon.com/...001GUZBZ2/