D90 is an awesome camera; probably a way too much for an introductory photo novice. I can ASSURE you , the most important part of close up photography (aka- macro/micro photography) is going to be the lens. A better option would be to spend about $500 on an SLR, $500 on a macro lens , and $200 on a ring flash that will diffuse light softly for close up photos and not blow them up.
Two great options from Canon and Nikon:
Rebel XTi (
http://www.bhphotovideo.com/c/product/613611-REG/Canon_3818B001_EOS_Rebel_T1i_Digital.html)
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Canon 100mm f/2.8 lens (
http://www.bhphotovideo.com/c/product/194451-USA/Canon_4657A006_100mm_f_2_8_USM_Macro.html) :used one myself for mouth shots, great setup
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Bower Ring Flash (Canon brand is just way too high;
http://www.bhphotovideo.com/c/product/630902-REG/Bower_SFD52C_SFD_52C_Dual_Intelligent.html)
Or for a Nikon setup....
Nikon D5000 (
http://www.bhphotovideo.com/c/product/615742-REG/Nikon_25452_D5000_Digital_SLR_Camera.html)
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Sigma 105 micro lens (much cheaper than Nikon alternative;
http://www.bhphotovideo.com/c/product/341922-USA/Sigma_257306_105mm_f_2_8_EX_DG.html_)
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Ring Flash (
http://www.bhphotovideo.com/c/search?Ntt=ring+flash&ci=643&Ns=p_PRICE_2|0&N=4294204370+4294955931)
Not trying to plug B&H, they just have tons of Camera gear and good information (as well as prices).
If you can't afford nearly that, then it looks like you'll just have to go out, bring a friend to an electronics store, and take pictures of their mouth close up using the digital cameras there. Otherwise, most P&S cameras will be about equal.