Chez Nikon, four is the new three.
The new four digit series, eg., D3000 replace the old two digit, eg., D40 series.
Thus, when I bought:
Pro - D2
Semi - D200
Consumer - D80
(I went with the D200 as the best cost/benefit balance. My son-in-law's D90, a newer generation, takes better images.)
Very few starving students need a D3. And if you need a D3, you know who you are. The D300 (APS sensor) and the D700 (35x24mm sensor) are Nikon's world beaters for the moment. The "consumer" model will:
-- Cost less because it is made of plastic vs. metal
-- Have fewer direct buttons and more menus.
-- Have consumer features like modes, etc.
-- Have cost saving features like a pentamirror vs. pentaprism.
-- Use SD rather than CF cards.
The key point is that the sensors and processors are generally the same - you won't see a difference in the image quality. Going to Africa or planning to get in bar fights? Buy the metal camera. Otherwise, you probably won't see the difference.
Any new Nikon will have MUCH better low light capabilities than the D100. (Nikon lagged Canon in this area until the present generation.)
You might find some time on these sites useful:
www.bythom.com (Thom, especially, discusses the philosophy of Nikon's product line.)
www.nikonians.org
Best wishes!
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