If you're a heavy browser user, or if you simply never close your browser, you'll frequently run into the scenario where 32 bit Firefox reaches the 2GB virtual memory limit, whereupon it gets killed. Yep, it's 2GB, atleast on 32bit OS (I'm on Win 7 32bit).

Just the convenience factor of not having to restart my browser frequently is enough to make me want to embrace x64. My blocker is that my corporate IT policy is to have 32 bit OS.