For the low graphics mode, the only solution is to install a video driver for your graphic card.
You can lower system resource usage by installing all XFCE4 packages including XFCE4-settings, and also Xubuntu-desktop and Xubuntu-settings packages.
This way you'll be able to choose the XFCE session (or Xubuntu session) at login prompt, and use fewer resources than you are using now when you can access only the Gnome session in Ubuntu. If you install XFCE4 packages dont forget to also install the xfce4-goodies package.
You can also choose to install all LXDE packages, LXDE package included, and also Lubuntu-desktop and lubuntu-settings packages. This way you'll be able to also choose the LXDE session (Lubuntu session) at login prompt. Lxde session uses even fewer resources than the Xfce session.
You can install all these packages using Synaptic Package Manager, and type in the Search field either xfce4 or lxde to find and install all required packages for each session. If you don't have Synaptic present on your systen, you can install it using this code:
sudo apt-get install synaptic
To make your system a little bit faster, I suggest you also install preload package:
sudo apt-get install preload
And also you can install bum package which will allow you to stop some daemons and services that you know you never use, like printers, scanners and so on:
sudo apt-get install bum