display problem on dell isnpiron 8000 laptop

I have a dell isnpiron 8000 laptop, pentium 3, 900mhz, 386mb ram and ATI Mobility agp4 card (I think this is the right name) with 16mb ram.
All work fine on the laptop with windows xp and other linux such as pclinuxos, or suse
with all ubuntu versions (including 7.04 beta) ubuntu starts up at 640x480 screen resolution (both live and once fully installed).
I can't set other resolutions, that one is the only option given.
The image is then garbled, with parts of the screen beeing repeated. Impossible to use the system and very difficult to go around and change settings.
The native resolution is 1400x1050.

It seems the auto-configuration is failing to select the proper settings for the card or the display.

I REALLY WOULD LIKE TO USE UBUNTU (DON"T LIKE XP OR OTHER LINUX) MAY YOU PLEASE ADVISE WHAT I CAN DO?
I AM NOT AN EXPERT ESPECIALLY AROUND LINUX SO WOULD BE GREAT IF THERE IS A SIMPLE SOLUTION
THANK YOU !