For the most part you could run any edition of Vista there since you have already met the basics with a fast enough cpu and having 2gb of memory installed. The differences between the 32bit and 64bit editions have more to do with the platform not the hardware requirements between editions if both for instance are the Ultimate or Home Premium editions.
Have any doubts? You can run MS's own upgrade adviser you download from
http://www.microsoft.com/windows/pro...a&mg_id=20409b But you already have more the 512mb minimum memory wise.
The largest problem seen at this time is seeing what programs will run on the 64bit edtitions as well as finding the 3rd party drivers needed. Support has definitely improved over what was seen for XP Pro 64bit however.