If you do use your backup camera a lot, upgrading the D70 to a D90 seems substantial - the D5100 to the D7000 not so much (unless there are specific funtions that would make your life better).I know it's not the question you asked, but if you do want to go FX at some point, I think you'd get more out of saving up for that (as long as you don't mind the added weight).If you're happy with your current equipment, I would think there are plenty of other things to throw money at : )
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