Recommended siganl for data if you were to perform a site survey is 0 to -72dbm. For voice it is recommended to be between 0 and -65dbm. So your signal of -59dbm is good. SNR = the signal strength minus the noise. So it looks like the noise level is 100 which is good. Here is a link with SNR:
Firstly during you should never let the signal go below -72dBm. From experience, if you are going to use WEP / WPA or IP wireless phones you should never let the signal go below -65dBm.
Then you should look at the SNR (Signal to Noise Ratio). That is the amount of signal you have above the noise. The higer the better.
Then lastly, whether you should choose range or throughput depends on what you need most. When configured for range the access point favours the slower bit rates, when throughput the access point favours higer bit rates.
1) The readings indicate that your signal is 41dBm above noise. I'm not sure about this but I think that to have a decent signal your SNR should always be above 12dBm. The higher the value of your SNR the better quality of your signal.
2) Your signal range being 0 to -72dBm has nothing to do with your SNR. Your radio strength is one thing and your SNR is another. Radio strength measures the amount of radio signal you are receiving. The SNR measures the diference between the strength of your radio signal and the strength of your noise.