I'm not positive, but this seems to be quite similar to another issue of late, that the version of the Nvidia driver bundled with the LMCE system is a bit old, and might not quite support your card yet. I'm in the same boat, with a Geforce 7300 GT on my LMCE Hybrid, but I got lucky, and the newest Nvidia driver actually loaded after a dozen or so tries... I imagine, though, that on a diskless, it will only send down the driver that is bundled. This, of course, is not good news, but I believe I read that it's being worked on.
Hey, at least you can get a diskless MD to boot at all... I can't seem to get one up to save my life...
Anyhow, good luck on it all!!!