Once you have done the suggestions below, you might want to look at the file
.xsession-errors
file in your home directory.
That usually has some indication as to what to do about things.
There's also a way to modify your
/etc/X11/xorg.conf
file to use the vesa drivers, but I am not that knowledgeable enough yet to
know exactly how to do this.

Another thing you could try is:

sudo apt-get remove gnome-session

sudo apt-get source gnome-session --compile

and
gdebi gnome-session.deb

that will compile the gnome environment on your own box.