I had a similar problem installing debian on a dell laptop. I changed the driver in /etc/X11/xorg.conf from ati to vesa. This solved the problem. It should work in Ubuntu too.
Here is where I found the solution in the Debian Forum.
Hope this is of some help
From: A. Belli [mailto:belli at iww.uni-hannover.de]
Sent: Fri 11/04/2008 08:27
To: ilug at linux.ie
Subject: [ILUG] Re: X on dell vostro 1000
Easiest think to do ist try ENVY:
It uses proprietary drivers though. Greetings.
Conor Daly schrieb:
> So, I got my new Dell Vostro 1000 laptop (EUR480) with 2GHz, 1Gb, 80Gb. I
> videoed the refusal of the MS XP EULA with the intention of returning the
> software for a refund. Then I installed a quick RHEL5 'cos I had a disk
> handy. Couldn't get X going. Downloaded a Ubuntu live CD (7.1 I think)
> and booted from that. X starts but I get a screen that looks like
> interlaced and out of sync.
>> It has a Radeon RS482 Xpress 200 card with a 15" widescreen LCD. ubuntu
> is using the radeon driver. I haven't tried the ATI driver and would
> prefer to use the free one if poss. Anyone had any luck with this card?
>> I'll grab logs and stuff in a bit.
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