What version of Windows do you have?
At the moment I have Windows Vista Home Premium (64-bit), but I may be updating soon to Windows 7 Professional (64-bit). I was looking at getting Visual Studio 2008 Professional, and it came in the two versions I mentioned earlier. I knew that x86 dealt with 32-bit environments while x64 dealt with 64-bit, but I wasn't sure what WoW was supposed to mean or what advantage there was to getting it. I'll remember in the future then to opt for x64WoW when I can, assuming there's no x64 version.