Any 64-bit edition of Vista except Starter (and perhaps basic) should be sufficient. I would think you would want at the very least Home Premium anyway... I don't think Home Basic is going to sell well unless it's preloaded on a lot of the big OEM's machines.
Like I said, stay away from Starter (don't think you can get this one anyway) or Basic. Any other version of Vista will have x86 and x64 "in the box". I'm just hoping that you will still have the option of installing 32-bit Vista just in case companies are still lagging with 64-bit drivers.