Reply to Fred:
The answer is simple. Insert your bootable 64bit Vista DVD, and once prompted, click Install
(formatting the hard drive is recommended but not mandatory).
There will be no other option.
I’m running both 32 and 64 bit Vista at my home on my two HTPCs (exact same hardware configuration), I find my 32 bit perform better with games. May be 5 years from now, every software vendor would choose to have just 64 bit applications and then, 64 bit OS will prove its power - not until then.