Q. Is there a performance advantage to using 64-bit Windows 7?
A. That depends. If you're working with large files or running applications that consume a great deal of memory, then 64-bit Windows will typically give you a slight performance advantage over 32-bit Windows running on identical hardware. This is true even when using 32-bit applications. That's because the OS and device drivers themselves benefit from the 64-bit wide registers of an Intel or AMD CPU running in 64-bit extended mode. In addition, because 64-bit Windows 7 supports more physical RAM than 32-bit Windows 7 (192GB on non-Home versions vs. 4GB for any 32-bit flavor), you can easily expand your PC's capabilities well beyond what is possible in a 32-bit world.
Q. Why does 64-bit Windows use more RAM than 32-bit Windows?
A. Any 64-bit OS will consume more memory than its 32-bit equivalent. This is due to the nature of 64-bit code: It uses larger internal structures that necessarily take up more space, both in RAM and on the hard disk. It's no surprise that the ISO image for the 64-bit versions of Vista and Windows 7 are roughly 50 to 70 percent larger than the equivalent 32-bit ISOs, or that 64-bit Windows shows 20 to 30 percent higher physical memory utilization after initial boot-up.
Q. Are there security advantages to using 64-bit Windows 7 vs. 32-bit Windows 7?
A. Yes. Many of the widely publicized kernel "hardening" initiatives Microsoft debuted with Vista are specific to the 64-bit flavor -- things like hardware-backed Data Execution Prevention and PatchGuard. Also, 64-bit Windows Vista and Windows 7 require device drivers to be digitally signed by their authors, making it tougher for hackers and root-kit developers to install their exploits covertly as kernel-mode drivers.
Q. Can I upgrade from a 32-bit flavor of Windows to 64-bit Windows 7?
A. No. Microsoft's upgrade process does not currently support moving between processor architectures. In order to upgrade from a 32-bit version of Windows you'll need to perform a "clean" installation of 64-bit Windows 7, then migrate your applications and data to the new OS.