http://www.ubuntu.com/support/faq
Particularly relevant for newer systems:

nVidia and ATI graphics cards

For the latest on these popular cards click here for nVidia or here for ATI.

How come I can’t use more than 900 MB of RAM?

I have more than 900 megabytes of RAM, but the system only sees the first 900. Why is that?

The default generic i386 kernel only has support for 900 megabytes of RAM. After installation, you can select a different kernel as appropriate for your hardware. If you are running on a P4, for example, you might type:

sudo apt-get install linux-686

Otherwise, if you are on a dual-processor workstation, you might type:

sudo apt-get install linux-686-smp

Choose from the list of linux packages the one that best suits your hardware.