I would strongly recommend choosing one of Nikon's newest models, either the D3100 or the D7000, over their predecessors. Both are best-in-class in the current Nikon line-up. The D7000 is vastly superior to the D90. The D3100, I'm not as familiar with, but I assume is better in many respects over the D5000, being newer. However, the D5000 does out spec it by a hair (4 fps vs 3 fps). Some features in the D3100 appear the same, and have not been improved in the newer model: both continue to use the same 11-point Multi-CAM 1000 AF, the same 420-color pixel metering, etc. But newer, typically does mean better. Another clear difference is that both the D3100 and D7000 have video quality vastly superior to both the D5000 and D90. Also, both newer models benefit from the faster processor.
But coming from a D70, almost anything in Nikon's current line-up will feel like a very impressive step-up!
Edit: I re-read your thread, and now I realize, the real dilemma may actually be choosing between a used D90 or a new D3100, since they're now about the same price. In that case, the D90 may be the better choice (however, if you think you may want to shoot video in any serious way, the video capability of both the D90 and the D5000 is so horribly flawed, that it's pretty much unuseable). Also, the D90 does have a screw-drive, and will focus any Nikon AF lens--both the D3100 and the D5000 do not--they are only capable of driving AF-S lenses.