In simple terms, a build is a sub-version/revision. So instead of V. 3.0.3 or whatever it would be Parallels just says Build 5160. You can move between them by un-installing the current build with the uninstaller you got with that build, and you can install a different build. Just make sure if you are doing this a lot you un-install Parallels tools prior to installing a new build, then install the tools once you have the new build is installed. Strange things have happened in the earlier versions. If you are upgrading, just use the installer.
As to the amount of memory, is a gig what you want to allocate? Unless you are using a program that eats up RAM in Windows, between 512-800MB tends to be the sweet spot in terms of memory. 1 GB of memory on the computer itself is a little small, and produces lag. I recommend 2 GB to people. 3 GB makes a day and night difference.
Does that answer your questions?
***Edited 09/22***
Error caught by dkp.
Last edited: Sep 22, 2007