^About 64-bit Windows: yes, it's going to be slightly faster overall by something on the order of 10%. The 64-bit applications will be faster on average, at least.
Talking of which: double and triple check the availability of all drivers for your hardware, as it takes specific 64-bit versions of them.
As for the lifetime of HDDs: it depends on the kind of use you make of them. If you just use them for the occasional backup, they are quite safe over the timespan of several years. If you keep them spinning all day long, not really. I went through 2 (150 and then 350 GB) HDDs connected to my file server last year, but that's because I had downloads and reads from my apache server going on almost 24/7.
External HDDs can be more prone to failure in the measure in which they're overheating: small external cases with tiny fans are again not suited to a 24/7 usage. But if it is a daily 10-mins backup process or your occasional copying of movies/photos/documents you want to lug around, then there's no reason to suppose their life to be shorter than HDDs mounted in your desktop computer.
