When you buy Vista you will get both the 32 and 64 bit version. so just do a reinstall in the future.
When I ordered mine, I had to specify 32 OR
64 bit. So you're saying it doesn't matter which version you order, that you'll get both? Are you sure about that?
Retail you get both, OEM you get what you ask for and can upgrade online (you cannot go from 64 to 32 bit though)