Can an x64 processor use 32bit apps?

Yes. x86-64 processors are capable of using both 32-bit and 64-bit operating systems and programs. Note that if you are using a 64-bit operating system, the system itself must be capable of running 32-bit binaries as well. Windows XP Professional x64 and 64-bit versions of Vista include this functionality. On Linux, since almost all programs come in a 64-bit version anyway, you have to install 32-bit libraries to run them.