The OP is gone by now, but I'll simply leave a link here to what I wrote in another thread: here. In this case the OP has a good selection of gear, and given that money is an issue, I'd recommend that she stay with the D40 for a bit longer. Bracketing can be done manually (and in fact it's quite easy), better high ISO can be attacked reasonably effectively by using software - and for a lot less money than a new body; and really I'm not that sure about how much more flexible the d90 and d7000 really are. They certainly have more features, but that's not necessarily the same thing (it might be to some people). $70 spent on noise reduction software can buy you a stop or two stops worth of NR - and for a lot less money than a D90 or D7000, although likely they will still provide a better overall result (especially if the software is applied gently to their files).
Per my comments in the other thread, I recommend hanging onto the D40 until you run into something that you really cannot do with the D40.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!