It's impossible to know how far you'll get with overclocking, there are simply too many variables. Even removing all but one variable; nobody can possibly know how much additional stress your particular CPU wafer can endure. The best anyone will be able to do is give a rough guess based on the success of others, however, no guarantee can be made. Trial and error is your best friend...
As far as a safe operating temperature is concerned; it's maximum temperature is probably around 70C-75C (though I admittedly haven't looked up the E8400's specifications), and you'll likely want to keep it below 60C-65C at load if possible.