I spent the weekend doing a complete migration of all data and programs from my old XP machine to my new Vista machine, not a single problem except for one program I use that has not been made "Vista compatible" yet, I got around that pretty easily by installing Windows Virtual PC, and loading up XP in it, works perfect.
Side note: both my new laptop and desktop came with Vista pre-loaded, so this may have something to do with not having any issues or crashes, whereas an older machine that was "made for XP" may not be 100% compatible with Vista, i.e. device drivers.
I am running both of them on only 1gig of RAM and they run very fast, I did switch them both to "performance mode" so they basically look and act like XP.