1st you have to make sure the machine will suport x64. If, it does, you can try using that key but, when you go to Activate you may (probably will) need to phone in to Microsoft & explain the scenario to get a new Activation #.
Ah, wait a minute... you have to use the key that goes w/ the x64 disc, then call MS to Activate. The x86 key will say it doesn't match the x64 SKU
Drew