Congrats on the 18-200, I've used that lens before and the results were awesome.I was very tempted to go for the SB-600 but for my current needs the SB-400 was more than enough. I'll upgrade in a year or so when I've gone as far as I can creatively with the 400. And go for the 50mm 1.4... you won't regret it
And as a sidenote, I've just had notice my 18-200mm VR has arrived in the post room.
I'm all giddy with excitement!
My next camera related purchase will be one of the Nikkor 50s... Once I get that, I'll be covered lens wise from 18-300, with a good selection for various tasks.
As far as flashes, my midrange Sunpack works fine, albeit with a slow recycle time. It's about comparable to an SB-600 which I've used. I guess for the price I got it for, I can't complain.