According to this: http://windows.microsoft.com/en-us/wind ... quirements
512mb of memory is the minimum to run Vista.
Now let me tell you that when Microsoft says minimum, they mean it will barely run. I wouldn't want to try with anything less than 2 GB of memory.
I wouldn't want to try running XP on 512 mb.
I'd say that is a little extreme friend, XP would be perfectly happy with 512MB. Even 256MB.
However, you are right on Vista, that thing is a pig dog and honestly isn't worth trying on that system at all.