1. Yes, the external flash is absolutely worth it, I'm 99% sure that's the same flash I use and love. Get yourself a diffuser (~$20) for times when it just isn't possible to bounce.
2. It's almost guaranteed the Tamron is nowhere near as good as the Nikon.
As for those that say the 18-200 isn't sharp enough, jesus christ you guys must be those that shoot full resolution RAW and then examine every pixel at 100% zoom on 24" monitors looking for a lack of sharpness. Or professionals, in which case the lens was never meant for you. For hacks like myself (and I suspect Todd) the gear is so far from being the limiting factor in our images it's laughable.