This is the one area where Burnsy and myself don't agree with regards licenses - so I will add my own comment because until I can confirm or see proof otherwise I'm going to stick to my interpretation of the license.
From the very fact that you've only got Vista Ultimate 32bit on DVD means that what you've purchased is the OEM version.
The Retail version of Ultimate ships with both the 32bit & 64bit DVD's.
You made your choice at time of purchase - you decided on the 32bit version of the OS and that is all you are licensed to use.
The retail licenses allow you to move between 32bit & 64bit as and when however there is no indication that the OEM licenses allow you to do this.
You make your choice, which you made and now you've decided you actually want the 64bit version you need to buy a 64bit license.
The link indicated here:
http://www.microsoft.com/windows/products/windowsvista/editions/64bit.mspx
Is for people who bought a version of Windows Vista
Retail so they can get a 64bit DVD because they are licensed to use it.
This is stated at the bottom of the page - notice it doesn't mention OEM versions because that isn't the case.
So as far as can be told from the license agreements you are stuck with the 32bit version of Vista until you buiy yourself a copy of the 64bit version.
Obviously this isn't the kind of post you want to read, so please don't "shoot the messenger".
I'm an IT Manager and I take licenses extremely seriously.
Until I can get something definitive to say differently it is a policy I stick to - there is no buying OEM Vista 32bit and then installing Vista 64bit - this requires two totally seperate licenses.
Before anyone posts about how you used the same key to install both 32bit & 64bit versions etc once again I need you to understand the difference between what is physically possible and what is legally possible.
I can go out, drink loads of beer, get stopped in my car and banned from the road for 12 months.
If I go out to my car the following day with my keys and attempt to drive the car anywhere there is nothing at all to physically stop me doing so.
However I would be breaking the law by doing so.
Anyway - I'll leave my post there.
Of course you won't want to really do as my post indicates as that would mean buying a new copy of Vista.
However remember that ignorance is not actually a defence with regards license agreements.