Yes norton will find PC virus'; that's all it can find - as no OS X ones exist.
As regards to x86 being more vulnerable as chip to virus' than PPC; that's not the case as you say, PC virus' attack Windows based weaknesses for the most part, so if Windows isn't there, a Windows virus won't work. Witness how you can get a virus on Virtual PC running Windows, yet really it's on OS X and on a PPC processor.
As such it's not x86 that's a weakness, it's Windows.
After all, how many virus writers these days are actually going to program in x86 assembler, and find an actual weakness in the CPU? FDIV was the last such example on x86 that was a hardware bug that caused crashes.
(FDIV allowed 7/0, 255/0 etc. which should cause a divide by zero error, on the few 60 and 66MHz Pentiums affected, it wouldn't and it'd crash).
As such being as no one programs virus' in x86 assembler, it won't suddenly mean OS X becomes vulnerable, as most programming these days is in high level code, meaning C++ which is used very prevalently for Windows application development.
of course if you manage to hack your Mac-intel to run Windows (hypothetically speaking) outside of something like Virtual PC then yes - bring on the spyware and virus attacks
Vicky