If you can get into Ubuntu using low graphics mode, which is likely a plain
vanilla nouveau driver, then you can install the required proprietary one
using System, Administration, Additional Drivers (in Maverick, in Lucid
called Hardware drivers). Choose the recommended one. If that does not work,
choose a recent one, 173 or 185. You can install more than one, but can only
use one at a time. You will need to re-boot to see any change.

If nouveau is installed uninstall it from Synaptic or equivalent.You
probably do not need to do this, but some distributions such as Fedora will
favour nouveau and as long as you have it installed it won't use the
proprietary ones. I don't think that Ubuntu does this, but it should not
hurt.