The best replacement for d40 is d80.

The best alternatives to d40 are rated as following:
★★★★★  d80
★★★★★  d40x
★★★★★  d60
★★★★★  d50
★★★★  d70
★★  d90
★★  d3000
★  d3100
★  d5100
★  d3200
★  nikon d60
★  d200
★  canon 400d
★  canon xti
★  xsi
  d300
  xti
  nikon d50
  canon rebel xti
  400d
  d22
  canon rebel xt
  450d
  d5000
  nikon d80
  nikon d300
  350d
  canon rebel
  rebel xt
  nikon d3000
  xt
  rebel xti
  d22 navara
  a200
  d60 nikon
  rebel
  fz28
  k100d

Your alternative for d40 is not listed? Suggest!
Speaking about d40, one probably shouldn't forget about battery grip.

Other search terms related to d40 and their ratings are:
★★★★★ battery grip
★★★★★ vr lens
★★★★ 35mm 1.8
★★★ 50mm 1.8
★★ sb 600
★ sb 800
You have in mind a companion for d40 and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of d40 in all years. Click on graph for more.
d40
d80
d40x