(note, sorry for the double post I'm fighting/experimenting with similar issues here)
If this was a desktop computer, where you get to use the GDM (login screen) you can choose a gnome session in that login screen; ie unity,unity-2d,gnome classic,gnome classic (No Effects).
If this is a headless machine and Unity/effects/etc are only getting in the way, ie breaking or slowing your VNC/RDP/NX/etc You may concider... cheating.
You could change the default gnome session used by locating where that setting is and changing ... Sorry, I have not found that (yet) BUT... I cheated!
The default session file is /usr/share/gnome-session/sessions/ubuntu.session
The plain old classic gnome without effects session file is /usr/share/gnome-session/sessions/2d-gnome.session
$ sudo su
$ cd /usr/share/gnome-session/sessions
$ mv ubuntu.session ___ubuntu.session
$ ln -s 2d-gnome.session ubuntu.session
drwxr-xr-x 2 root root 4096 2011-06-09 00:14 ./
drwxr-xr-x 3 root root 4096 2011-05-31 00:34 ../
-rw-r--r-- 1 root root 202 2011-04-26 23:55 2d-gnome.session
-rw-r--r-- 1 root root 247 2011-02-24 13:37 2d-ubuntu.session
-rw-r--r-- 1 root root 303 2011-04-26 23:55 classic-gnome.session
-rw-r--r-- 1 root root 571 2011-04-26 23:55 gnome.session
-rw-r--r-- 1 root root 606 2011-04-26 23:55 ____ubuntu.session
lrwxrwxrwx 1 root root 16 2011-06-09 00:14 ubuntu.session -> 2d-gnome.session
It's quick, it's dirty, it does the job.
Any filename would do just so we can make a symlink for the default session's filename.
The proper way would be to change the config file above the session (ie the one that calls the session).
If someone knows/can find the location of the config file that has the default session's filename in it please do share! :)