plus, i thnk D40's performance won't lose out much to D90. get the 18-200 VR. its better that way unless you need the video function.
While it is possible that you can make do with the D40 at the moment, I choose to disagree that D40 and D90's performance are not far apart...
since the TS has mentioned that you do a fair amount of relatively low light shots...and prefer natural lighting...perhaps higher ISO performance would also suit your needs...(this complicates your decision doesnt it..)
Point taken taken that photographers ages ago who had no access to current age technology still were able to get shots..and many of them did so with technology less advanced that what D40 has to offer...but this is a fact..that technology is improving each day...are we never going to embrace it..?
At the end of the day, if u stick with your D40 and were using the 35mm f/1.8, there may come a time when you may miss 'the shot' because the D40 was limiting you in terms of ISO performance...(MAY....do you think so...?)
If you are willing to spend the money, my vote is for an upgrade in the body as well...