The WEI score is for the LOWEST performing item in your system - typically the hard drive. You also need to clear the existing test data before re-running WEI or you'll get the same score regardless of changes.
The FX-8320 will run faster in some applications and about the same in other applications to an equal frequency PII CPU depending on how well the software is written. Some games will run up to 50% faster with an 8-core FX processor over the fastest PII CPU.
There are many variables involved including the drivers you are using and the operating system. Win 7 has a "fix" that results in an average of ~5% improvement for FX processors vs. when Win 7 was originally released. Win 8 is reported to provide another 5+% improvement over Win 7.
Those of us who have run PII and FX processors back-to-back can advise that there is a nice performance bump with the FX 8-core CPUs over the PII CPUs. How much boost in performance you get varies based on the O/S, drivers and software. Some Linux folks have reported a real nice boost with 8-core FX processors also.
Building a reliable PC involves more than just assembling the parts. You need to be able to configure all of the BIOS settings appropriately. This can be quite involved and frustrating as it can require a lot of trial and error with stress testing. It is however often the only means to get a 100% reliable PC.