do i get another 6800 and go sli?
No! 6800 is too old now.
do i upgrade the gpu to a monster?
I would. I would get an 8800GT 512MB and be done with it. £170 or thereabouts?
would upgrading the gpu render the cpu useless?
Don't understand this one. That CPU will be a bottleneck to a certain extent, but it's hardly decrepit.
do i sell this machine and start from scratch?
How much do you think you'll get for it?