So you wanted to change it to 64 bit only due to get full RAM usage, right?
I also had such thing once I upgraded my RAMs to 4GB 2 years ago.
You can only install the x64 version of Vista if you have a 64-bit processor.
So first check if you have 64 bit processor.
Using 32 bit programs under 64 bit is not possible.
Having two vistas is also not possible may be using some pre boot softwares and on different partitions. ( I havent tested it)
Daemon tools is a thing to create a virtual drive and load the image files on that if you dont want to extract them. I dont know what use you want to make of it.
go aahoo!!