This article contains a few tips for model authors who create 3D models to be used in place of the original sprites.
When creating models to replace an object in the original game, try to achieve as good resemblance as possible. The original sprites are a good reference, you can view them for example with DeePSea. Comparing models vs. sprites is easy if you use bindings such as these:
The recommended triangle count for monsters is 500-800. Use more triangles for larger, detailed objects. Small objects should have less than 100, so there can be lots of them with no problems. Remember that lighting usually smooths out a model quite nicely.
Use as small skins as possible. There should be no need to use skins larger than 256×256. Remember that you can position the texture coordinates yourself so you can optimize the usage of the skin image.
If you preview your work in the game, make sure you've disabled all software gamma correction (“setgamma 0”) and set a good display gamma (vid-gamma) so your model won't appear too bright or dark for others.