Retail Ultimate comes with both versions, 32 and 64. OEM comes in either 32 or 64, not both, so you must choose before purchase.
As for which to get that will work with your software - ALMOST everything will work on Vista 64. Even if the software is 32 bit it will run on Vista 64 in 32 bit mode just fine. There are a few exceptions to this rule here and there. Not sure about your software but if the software maker says get 32 a bit OS then they probably know what they are talking about. But be sure you get the authoritative word from the makers themselves because there is a lot of misinformation about 64 bit floating around on the web.
Main advantage of 64 bit at this time is the ability to use 4 or more gig of RAM. 32 bit MS oses are limited to about 3 gig +\- , for complex reasons.