Hi ADB,
Welcome to Vista Forums.
The retail box of Vista Home Premium will only contain the 32 bit version of it, but you can order the 64 bit version DVD of it from Microsoft for just the $9.99 S&H fee. You will use the same product key number from your 32 bit version for the 64 bit one. However, you can only have one version installed at a time on any computer, not both at the same.
You can order the 64 bit DVD from here:
Windows Vista Alternate Media
Hope this helps you,
Shawn