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.
So, for anyone with a Pentium D chip, just FYI, we were still having problems with the 32-bit install of 7. I finally started taking some other things out of the equation, such as the new RAM I bought for the upgrade as well as the video card. I changed out the new RAM for the old RAM she had in there and 32-bit ran without any issues. Obviously there was an issue with one of the simms.
Knowing that I couldn't run the 64-bit with only 1GB of RAM (what she had in old RAM), I went out and bought two new 2GB simms and put them in and reloaded 64-bit (build 7229 - which is what I have on my machine). It's been running now for a day and a half with no issues.
So, bottom line, Pentium D CAN run 64-bit. All along this was a RAM issue...
I have been a long time lurker here, but I thought I should post and clear up some issues as my current job is ensuring 64 bit software compatibility across a large corporate network in the UK. If you have any specific questions let me know and I will do my best to answer them.
First of all, to check that your CPU is x64 capable there is a handy utility from VMWare which does just that (for both Linux and Windows).
Or, you can look up your CPU on the 64 bit wikipedia entry which I help maintain. It contains lots of useful information on the technical changes in x64 and is worth reading if you are interested in this sort of thing
There is a useful guide to how you should decide between 32 and 64 bit versions of Windows 7 which I agree with. But I would also add that it is at least worth trying x64 Windows 7 RC if your system is capable of it, as you can always swap back to 32 bit later when you need to do a full re-install anyway.
If you have any questions about software compatibility do let me know and I will try to help.