What should be the threshold for load average of a quad core processor? What constitutes "good" and "bad" load average values?
Depends on what you're running....disk intensive operations will have different tolerances than memory intensive...
Point to note - The load average really relates to run-queue length (i.e sum of processes waiting in the run-queue plus those currently executing), so it's more about what still needs to be done, than what is currently being done...
Having said that, 
short answer is that general consensus is that a load average of 4*number of cpus is considered reasonable.
Adrian Cockcroft wrote a couple of excellent books on performance tuning - highly reccommended reading...
HTH
May i know the name of the books. Thanks