Hi Anand
I dunno if this helps since I don't use Ubuntu.I've had similar problems on my Debian unstable.I'm using the GE Force MX 400 too.
While I don't recall the exact steps I took to get things working I think the problem lay in the fact that Debian was making changes in their C/C++ ABI support. Therefore compiling and using the latest NVIDIA drivers was giving me probs for a very long time.I was only able to resolve things once Debian stabilized with their ABI transition and l compiled the latest driver from nvidia.
Since Ubuntu is basically Deb Unstable maybe what you're using still has some of these issues.Try using the latest version of your distrib and compiling the latest drivers and yes one thing I can confidently say is that the MX 400 is supported by the current drivers.Check the docs nonetheless.
Cheers Jeetu