virtualbox.org

Until Tuesday, all my VB experience was entirely with the 32-bit version (because I was running Vista 32-bit as a host on a P4 Northwood-C). Now that I've gone and migrated to an x64 CPU (for now, a Celeron DC E1200) and x64 OS (Vista Ultimate x64), I'm also migrating to VB x64. What particular *gotchas* should I be aware of in using VB x64 (I'll be sticking entirely with x64-based guests)?

There are no catches. You can run 32 bit Guests just fine. You can only run 64 bit Guests if you meet the hardware requirements mentioned in the manual.