I did quite a bit of research into flash guns, and just when I thought I had it worked out came across another brand/model that would work and had to start all over again in comparing them and what would work for my needs and also some future proofing without spending just as much as the camera cost
In the end I decided I didn't use flash that much for what I do, mainly outdoor work such as landscapes or long exposures, so put the idea on hold for the moment. But taking shots in the local caves did show I have a need for 'some' flash but managed with the built-in one
It also depends a lot if you want a manual flash, semi-automatic or fully-automatic
Manual flash guns are very cheap, but you have to do all the work. There are some very good looking budget fully-automatic flashes that seem to fit what I need which may be a good starting point for you
I was leaning very heavily towards either of these :
Yongnuo 465
Yongnuo 565 EX
If you look hard enough you can get them for less than the price of the Nikon SB-400. But the SB-400 is very very small compared to these two. But there are full TTL flash guns and better than the SB-400 for less money
Well, that's the conclusion I came to without spending almost as much again as my camera cost ... well cost the wife for Christmas last year