I think you should treat 32-bit and 64-bit in the same way. So, I suggest you add 64-bit support in your free edition.
From Windows 8, there will not be 32-bit any more. So, you have to add 64-bit support in your free edition one day in the future. And I suggest you do it soon.
If your free edition supports 64-bit, you will have more customers and I think you will make more money. If your free edition do not support 64-bit, those 64-bit users will surely use other softwares instead of paying for your Standard Edition. You will not make more money, I think.
This is my suggestion.
Thank you very much!