i don't know how i missed this one...
Ant, I would just keep the road bike you've got. There's no need to upgrade to a carbon frame. Yes, it will be stiffer and will put power to the groud, but unless you running competitve criterions, etc, you're just going to be throwing your money away. The difference will be marginal, at best. In fact, it will make your ride more uncomfortable because of the stiffness of the frame.
And one thing you have to look out for on higher end road bikes is weight restricitions (I know this because I'm over most/pretty much all of them). Being over 180 lbs will often void warranty on higher end parts.
DEFINITELY get a new MTB though. I finally got out there last weekend with Jay and Carl and it was hella fun. It's completely different from road riding and hella tough, but a complete blast and much more intense from a holy-shit-imma-crash perspective.
Good luck with the move!