Don't have a 3100 or a 3200 but can answer the question about how long before you want to upgrade. Three days is a good figure. Bought a D5000 and within a couple of weeks the D7000 came out. Fought the NAS for about 3 months but caved in and bought the D7000. Along comes the D5100 and I thought it would make a much better back-up than the D5000. Fought the NAS for a couple of months but again caved in and bought the D5100. Still have all three cameras. Now the D800 is calling my name pitifully. However, at this point, if I bought it, the wife would reach into my chest and rip out my by-passes. What a shame.