Vista runs fine on 1 GB RAM. There are several potential reasons to run Vista over XP. However, for the two scenarios mentioned -- GPS and customer database -- there is no particular reason to prefer Vista over XP.
GPS navigation software does not require a high-performance computer. Your choice should really be determined by other factors. Since you seem to like the Acer Aspire, you may want to check out a board dedicated to it, like:
http://www.aspireoneuser.com/forum/