"1.) Is a D90 to D7000 jump worth it in terms of low light performance and dynamic range?"
While the D90 is a great camera, the D7000 does have improved low light capabilities. I can shoot 1600 or even 3200 ISO and feel comfortable in most cases. It also has several improvements over the D90 in addition such as dual card slots, 100% view finder, etc.
"2.) Would it be better to skip the small jump to the D7000 and try to get into FX if I can, even if I need to shoot mostly in DX mode for a long time."
The D7000 jump would be less financially traumatic, but you may want to consider something other than the 18-200 DX lens. Some users have reported that the 18-200mm appears soft with the D7000.
Shooting in a DX mode on a FX camera for an extended period would not be fun. Invest in glass over time and then make your move to a FX camera.
"Red is gray and yellow white, but we decide which is right ....and which is an illusion"