Depende kung anong gusto mong gawin, no? General photography or do you have any specific interests? Anong budget mo?
Kung bago, they all really have the same sensor, except for the D3X - 12MP, so that's not a defining feature.
Medium class - well, that could put you in D90 or D300 land. Clear differences in build and features - pero defining difference is of course the ability to shoot video with the 90.
12MP is large, so I wouldn't go with anything less than 4GB cards. Stick to Sandisk and Lexer kung pwede. Reduces the uncertainty of being compatible. If you're planning to shoot video with the cards, kailangan Class 6 (which is an entirely different rating system than the 80x, 133x, etc).
Start with the lens that comes with the body if possible. Then usually people go two ways, bibili nang telephoto or an external flash. Kung telephoto, madaming choices, or meron kang lumang Nikon lenses, baka compatible pa. Kung flash, kailangan iTTL compatible para gagana nang lahat nang features, so that makes it an SB-400, SB-600 or SB-900. Kung meron kang makitang SB-800 on the used market, pwede rin.