if i was to shoot this, and im not using full manual:
id shoot aperture priority. then bracket your shots about a stop to a stop and a half darker.
i never like to go higher than 800 but i dont know d90s high ISO handling so i would guess 1600 is still alright since its a decent body.
id would also strictly shoot at the wide end of that zoom lens as it would have the faster aperture setting.
(this would concern you if im going full manual) as for shutter speed, 1/30 the slowest... id push it down to 1/10-1/15, obviously with very still hands.
im never the one to use flash so id leave that for someone else.
beside this one thing that i really like about flash photography.. slower shutter speed and 2nd/rear curtain. always an interesting shot
WB, its up to you to have an incandescent glow for a warmer vibe or have all the whites, white.
choose your WB setting accordingly.
i also like to set my exposure metering to spot or center so i wont have say a light source in the background telling my camera that the scene is over exposed whle it makes my subject way under exposed.
i strictly shoot with center AF point only. i dont like the fact that sometimes, the camera would lock in to something else in the scene and ill lose that shot trying to refocus.
point the center AF at your subject, half press to lock focus and recompose to shoot.