Help with overclocking AMD PII x3 720

Usually helps to post your hardware when you're asking for overclocking help. Just adjust your multiplier with your BCLK at 200, and you should get to 3.4 GHz without upping any voltages. Past that, you'll have to up your VCore and memory voltage, depending on the type or memory you have.
 
You should be able to get around 3.7 ghz going out of stock voltage, depending on the quality of that particular batch.. Now as far as life expectancy of processors, overclocking doesnt normally shorten it's expectancy if done properly.

Things to keep in mind while overclocking, for life expectancy risks:
1. Voltage (keep it below the maximum voltage indicated by the manufacturer!)
2. Temperature (Temperature usually only matters past 50c for prolonged usage, in which case your CPU could wear out)
3. Depending on the quality of the processor, since it varies from CPU batch to CPU batch. A good processor can usually output 15%-18% increase in clock (GHz), under stock voltage (v).


If your processor requires a bit of voltage, say 1.45v for just 3.2 ghz then I would NOT recommend overclocking it because of temperature spikes as well as instability.