Unless you have more than 4GB RAM there's not really any significant performance gains. Native 64 bit apps can run a little faster (because the x64 instruction set has more registers than x86). Most 32 bit apps will run the same or slightly slower on an x64 OS.

It is impossible to install 32 bit drivers. If there are no 64 bit drivers, you cannot use the device in a 64 bit OS. Furthermore, a 64 bit process cannot load 32 bit DLLs; this means that since explorer.exe is 64 bit, all your shell extensions must also be 64 bit, otherwise they won't work (this is also the primary reason why Windows x64 also includes a 32 bit version of Internet Explorer, which is in fact the default; the 64 bit IE cannot load 32 bit ActiveX controls).