Cache size really only makes a notable difference when your CPU is accessing the same data over and over again and reusing it...so stuff like rendering and A/V work would really benefit from a larger cache size noticably
AMD64 architecture has been designed to make effective use of its cache therefore you won't really see that much of a difference between 512KB and 1MB L2 cache...although the jump from 128KB to 512KB would probably be more noticable
If you're talking about actual 64 bit instruction sets themselves, cache size will start to benefit...64 bit registers are much larger than 32 bit registers therefore they'll take up more cache/RAM space and you'd notice a 32 bit process would take up less cache than a comparable 64 bit process