If you have the Retail version, Microsoft will send you a 64 bit CD for something like $10 (handling charge). If you have an OEM version of Vista, you're SOL and will have to buy a new copy. This is the same licencing structure used for XP.
Please be advised that you cannot "Upgrade" your existing OS from 32 to 64 bit. Going to 64 bits requires a clean install. Vista will auto-detect the existing installation, and will put your data into a folder called WINDOWS_OLD. Now... If you - like most people with any reasonable amount of intelligence - would put the phrase "Microsoft will save your existing data...." into the same general category of trustworthiness with other phrases, like "Honey, I *swear* I won't C*m in your mouth...", then you will obviously back up everything and make VERY SURE you capture all of your existing software licences for your aftermarket eMail client, MP3/4 program, CD burning software, etc etc etc.