For most the $85 difference in price for Vista Ultimate ($180) is not worth the few extra features over Vista Home Premium ($95).
You can see the differences in versions here:
Which Vista Edition is Right for You? You'll note there are 32bit and 64bit versions of each.
In general you'd go with 32bit version if you have 4GB or less of RAM and have older, legacy hardware and/or software to carry forward.
If you have a new system and recent software releases, may use 4GB or more of RAM you'd want to use the 64bit versions.
The 32bit version is limited to 4GB of address space for all requirements including RAM and the 64bit versions have the larger address space available.