during the config DB creation when you’re doing the install process, we’ve seen the memory consumption peak around 900MB… if you don’t have enough memory, you’ll get an OutOfMemoryException and the installation will fail.
I’ve been able to run my MOSS B2TR VM’s with 1024MB or RAM with no problems (my installs also don’t have AD created, but do have VS 2005, all the Office 2007 clients installed, and SQL 2005 with all the extra add ons [SSIS, SSAS, SSRS]).