The one product key (or bios SLIC) activates both versions, x32 or x64 bit. If you need a x64 bit 7 DVD, Microsoft is supposed to send it to you upon your request at low cost, significantly lower than $200, smth around $30 if I remember right.
Another option is to contact your PC vendor to ask for x64 DVD or ask for a refund - I understand they sold it as x64 preinstalled, that's what you paid your money for, correct ?