I generally fall in the 'spend money on lenses not bodies' camp as well. That being said, I was somewhat 'forced' into upgrading my D40 to a used D80 a few years ago to be able to use the 50/1.8 that my mom was kind enough to get me for christmas, and that I hadn't done the research to notice wouldn't autofocus on the D40.
I was actually pretty surprised at how much I appreciated all of the extras on the D80 vs. the D40. In particular, all of the dedicated buttons made it much easier to experiment quickly and on the fly. The use of the 50mm prime made the biggest difference in my pictures, unquestionably, but not having to hunt around in menus to try different settings shortened the feedback loop in terms of my own learning how to achieve different effects I was trying to get to. I have big hands, so the grip was a lot more comfortable as well, and I enjoyed shooting with it a little more.
The D40, like the D3000 is an extremely capable camera, and I would say is going to give you pretty much the same results as the D80/D90 with the same lenses. Having learned as much as I have by using a D80 for quite some time now, I actually think it would be easier for me to get the same results out of the smaller bodied cameras, as all I'd need to learn is where the settings reside in the menu to get the effect I'm used to using dedicated buttons for. But it was much easier to learn with all of that right at my fingertips.
The only thing that bothers me about the D3000 is the rather long delay between taking a shot and it appearing on the view screen. That would drive me crazy if it couldn't be mitigated.
I just bit the bullet and upgraded to a used D90 being sold locally here on craigslist. $570 with a 4Gb card and the remote shutter release. I'm hoping to sell my D80 for around $375. For me, the question came down to, 'would I pay $200 to make my D80 have a bigger, nicer LCD, more consistent matrix metering, and better low light performance? Those are all things that actually felt like they'd help me get shots that I'd missed, or make my photography tangibly better. They were my only complaints with the D80. It's hard to imagine them building in more features to the new release that I think could actually help my photography in the same manner as the D80->D90 upgrade, and certainly not in terms of bang for the buck. So I can imagine staying with the D90 for a very long time and only spending money on lenses if anything. It's a mature platform, in my estimation.
What's to take from all of this rambling? Probably not much, except to say that I think the D90 is a pretty sweet spot both from a learning and using perspective, and if you were to keep your eye out locally to pick one up used on the cheap, it would be a solid upgrade for you that would last you for a long time. I wouldn't feel badly in this case spending a few hundred dollars in this case to upgrade to a used D90 body vs. spending the money on lenses.