Well for starters, the LN line is not LED. That's just regular backlighting, CCFL I assume. So I guess you can cross that off your list if you're into an LED LCD TV.
The difference between the series 6 vs series 7, as from a Samsung press release,
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Building on the superior technologies and features found in the Series 6, the Series 7 takes networking and multimedia management to a whole new level. Consumers can easily enjoy MPEG and JPEG files stored on external devices by connecting through a side-mounted USB 2.0, turning their LCD into a full-scale home viewing gallery, or connect their MP3 players for a dynamic audio experience.
Additionally, the Series 7 features internal flash memory loaded with premium content to enjoy right out of the box, as well as an integrated woofer for an enhanced audio experience.
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So if you don't absolutely need the multimedia upgrade from the series 7 then you can save some coin and just get the series 6 machine which is equal in picture quality. If you look for it on Amazon its listed at exactly your budget limit.
The Samsung 40" LNB650 is basically the same as the UN line except CCFL backlight. So the blacks are like more typical LCD blacks (not black). But otherwise a very solid set. And noticeably cheaper (like $800 less).
At $2100 you'd be better off getting a high end like 50" Plasma. Its better image quality than an LED LCD still. The LED LCD makes huge strides for LCD but it doesn't knock of Plasma sets.