Target Bitrate for rendering a 720x480 movie while maintaining the highest video quality?

bitrate is not as important as the quality of the original source. if you have low quality footage, you'll need higher bitrate to compensate for retaining what little you have. you'd have to test various settings unless you want a rough guide in handbrake with constant quality at 18 and let it decide how much bitrate it needs. quality is determined by the bits/pixel ratio. there are lots of calculators out there, that's why handbrake made constant quality to separate the generation loss from being a mathmatician. 

Constant quality vs bitrate

1. Faster Encoding - No need to do 2-Pass encoding

2. Reduced file sizes (The latter requiring less bitrate to achieve a set quality level)

3. your overall output filesize is reduced without any loss of quality.