The 64-bit version of Live allows the use of more than 4 gigabytes of RAM. This means that Live users can now work with Live Sets (including Sets using third-party plug-ins and ReWire configurations) that are much larger and more memory intensive than in the 32-bit version.
The 64-bit version of Live works with PCs running Windows Vista/7/8 (64-bit) and with Intel-Macs running OS X 10.5 and up. The 64-bit version of Live 8 does not support Max for Live, video, or The Bridge. Ableton is currently working with its partners to add support for these features.
The release version of Live 8.4 will be available later, as a free update for registered Live 8, Suite 8, Live Intro or Live Lite 8 users.
Details about 64-bit operation: ableton.com/what-difference-between-32-bit-and-64-bit-version-live