So, in this little benchmark session, we see the great impact that 64 bit processing (in the form of X86-64) and SIMD paradigm (SSE2 extensions) can have on number-crunching applications, for example encrypting / decrypting programs, as John the ripper.
While often snubbed, these IA extension can pay off very bid dividends in proper applications.
I'll repeat: don't expect this great performance gain in general day application. In these common applications, the only thing that can really give you a little boost is the X86-64 extension, with its added registers. But if you use some heavy applications, as audio / video renderer, encryption / decryption software, CAD / CAM application, be sure to use a 64 bit, SSE2 enabled program.
Obviously, the better 64 bit capable Intel processors is Core i7, but I think that also an high-clocked Core 2 CPU will not disappoint you.