Ok thanks for the suggestions.
The AMD 7750 looks like a good choice at this stage - I might see if there's a fanless version of it (or get something around that mark, I would only ever need to go to 720p internal IR at most).
Its a custom made case (all made by me
) so I think the length of the 6850 is going to be a problem anyway... and just by looking at it, that thing would sound like a jet plane compared to the rest of the system 
The AMD 7750 looks like a good choice at this stage - I might see if there's a fanless version of it (or get something around that mark, I would only ever need to go to 720p internal IR at most).
Its a custom made case (all made by me
) so I think the length of the 6850 is going to be a problem anyway... and just by looking at it, that thing would sound like a jet plane compared to the rest of the system