If you've currently got Windows 7 32-bit, you may not need to buy a new copy - you could just download an ISO of the x64 version of the same edition here ->
http://www.w7forums.com/threads/official-windows-7-sp1-iso-image-downloads.12325/ - note the link to the SP1-U Media Refresh - as your existing licence key should work. (I've done it for a number of people who bought computers with 32-bit installed then wanted to upgrade beyond 3GB RAM.)
If you don't currently have Win7, or you don't have the CD key any more, you need Win7 Home Premium unless you want to use more than 16GB RAM, in which case you need Professional.
Hope this helps.
Stephen