RAM

• Any 32-bit Windows operating system is limited to a maximum of 4GB of RAM. With 64-bit Vista, it's possible to use up to 128 GB--but it depends on the version. You can reach the 128 GB maximum with 64-bit Vista Ultimate, Enterprise and Business. If you have 64-bit Home Premium the limit is 16GB. If you have 64-bit Home Basic the limit is 8GB. Microsoft recommends a minimum of 4GB to run any version of 64-bit Vista. Technically, 64-bit Vista can run more than 128 GB of RAM, but most computers don't have the ability to hold more than that yet.
  
  

CPU

• Running 64-bit Vista requires a 64-bit (x64) processor with a minimum of speed of 1-GHz. If you don't have 64-bit Vista and want to know if your CPU meets the requirements, right click on the "Computer" icon, click "Properties," then click on "Windows Experience Index." Select the link that says, "View and Print detailed performance and system information." It will tell you if your CPU is 64-bit compatible.
  
  

Compatibility

• Programs and devices originally made for 32-bit Vista should work fine in 64-bit Vista. If you run into problems it's most likely because you need to load a 64-bit driver--32-bit drivers won't work. But you can't run 64-bit software on 32-bit Vista, nor can you run 64-bit software on a 32-bit Vista machine that has a 64-bit CPU. You need both.
  
  

Security

• Security is stronger in 64-bit Vista. It comes with what's commonly called PatchGuard, which prevents malware from patching the Windows Vista kernel. Microsoft also strengthened its Data Execution Prevention feature. DEP protects against buffer overflow, which is a common method attackers use to hijack program code.
  
  

New Standard

• Sixty four-bit computing is the new standard. The number of companies that don't make 32-bit versions of their software any more keeps growing. For example, in 2009 Microsoft started producing its server software (Windows Server 2008 R2) in 64-bit only. There are now 64-bit Internet browsers. Of course, you don't have to buy 64-bit Vista or any other 64-bit OS or machine. But it won't be too long before 32-bit is relegated to the history books.