You can get the upgrade for 64 bit(32bit xp is eligible for 64 bit Win8 upgrade). But you will have to do a complete install, you can't just upgrade the OS from 32 bit to 64 bit. This means you will have to backup all your files, install the new OS, re-install all your applications, and restore your files.
Yesbut...if you choose the Download option as opposed to the 'Send Disk' you are at the mercy of the Dreaded Upgrade Assistant, which will inspect your system and upgrade 32/32 or 64/64 depending on which you have. As The_Prophecy says, if you want 64 bit from 32 you has to buy the disk(s). Until Legal Downloads become available from Digital River of course...