Unity Desktop not loading

[G.M.Hodson]

After upgrading to 11.04 the first notice I received on restart was
'insufficient hardware to run Unity Desktop' (or words to that effect)
My processor is AMD Sempron 2800+ running at 1.6Ghz, I have 873 MiB of
RAM, my multimedia card is NVIDIA & I have a 163 GiB harddrive of which
14% is used. I am booting from XUBUNTU but use Gnome, What can I do to
get Unity to load? I appreciate the biannual Upgrades from Ubuntu but I
would like to enjoy full use.