DICE has made considerable improvements over their Frostbite engine with the release of Battlefield 3. Consequently, these improvements and enhancements demand more power from your PC. According to Johan Andersson, rendering architect at DICE, some Frostbite powered games in 2013 will demand a 64-bit operating system.
Currently, PC gamers can run popular titles fine on a 32-bit OS, which makes convincing them to switch difficult. The real benefit gained from a 64-bit operating systems is that it can address 4 GB's or more of RAM. He states that the main driving force behind this requirement is larger Virtual Address Space (VAS) and RAM access.
Johan Andersson suggests that if you do not currently own a 64-bit OS, the release of Windows 8 is a good opportunity to upgrade.