I know this question has already been answered, but I just wanted to provide another demo for how limiting 42mm is. (42mm = 28mm*1.5) Just pull out your old N80 and the 28-105. Set the lens to approximately 42mm (just down a bit from 50mm, roughly equivalent between 50mm and 35mm). Now walk around a bit and imagine that that's as wide as you can go. It is pretty limiting. There are some situations where you can just "back up a few steps" but in many cases there is no way to get that additional angle of view that a true wide-angle would provide.
I would recommend a D80 and an 18-55 kit lens. They are both pretty cheap at this point. The 18-105 has a lot better range, but you've already got the longer focal lengths covered with your 28-105 and 70-240. If you are comfortable with spending bigger bucks for the D90, it's a great upgrade, but maybe not worth the cash for someone who's just getting their feet wet.
Used camera equipment (esp. Nikon and Canon) is very easy to buy and sell. I currently have two bodies, but I have bought and sold 6 so far. Each time, I worked my way up a notch and added a couple hundred bucks. I have tended not to churn my lenses as much. If you buy a $600 body and use it for a year before deciding to upgrade, barring major damage you should be able to sell it for $450-$500. So effectively you "rented" the camera for $100 for a year. Not bad at all. Lenses hold their value even better, since Canikon updates bodies yearly but lenses not so much. If you buy a used 18-55 for $100 then it is pretty much a guarantee that you will be able to sell it a year from now for $100.
So if you are value-conscious it is quite easy to treat your camera equipment as an investment and to lose little minimal money on it in the long run. (In some cases you could even make money if you are a careful buyer, and if unfavorable exchange rates from Japan continue to raise the price of new lenses -- although this tends to affect the big, $2000+ lenses more than the consumer stuff.) First rule, don't buy new. Second rule, get on the big forums (photography-on-the.net and fredmiranda.com) and start establishing a reputation as a trader.