I'd like to see a source saying Pentium D is any way less 64-bit than Core 2. Intel says Pentium D supports Intel 64. The Wikipedia articles on Pentium D, Core 2, and x86-64 say nothing about Core 2 being 64-bit in a different sense than Pentium D.
Either way, you would just need to buy Windows 7 once, then fill out the order form for the 64-bit DVD and pay shipping and handling. You would get the 32-bit DVD in the box, but I'm sure you wouldn't need that at all to run the 64-bit version.