Quick fact from checking dmesg and other kernel info, once I rooted my Atrix.

The phone has 1GB of RAM, but upon boot it is allocated like this:

mem=320M@0M nvmem=192M@320M mem=512M@512M

Translation from kernel args to popular language:

RAM available for the OS: 832Mb
RAM reserved for the "video card": 192Mb

Even after a clear boot, with webtop completely stop, you can run /usr/bin/free and notice that there is indeed only 832MB of RAM available to the OS.

And as I posted on the the other thread about the webtop, there is no RAM consumed by the webtop when you are on "mobile mode" only. Webtop is just a second X session started on the linux OS that is below Android, redirected to the HDMI port.