Its been a while since I've bought a new GPU & I'm just about to switch my 4850 for a 7870. Latest catalyst drivers installed, am I right thinking that I can just switch the cards & don't have to uninstall followed by reinstall the drivers? There, told you it was basic!
I'd uninstall, then reinstall - I had a problem when changing a friends hd5600 series card to a hd6850. Once I'd uninstalled the drivers (with the 5600 back in), I was able to install the new card and drivers without a further problem.
Always best to do a full uninstall of driver. Put in card, reinstall driver. Just saves you from any random issues that may crop up due to incorrect settings. Will take all of 10 mins.
From my own experience with AMD cards, their drivers can be fussy if you just swap in a new card. Nvidia is better in that respect. But yeah, uninstall the drivers and reinstall. It'll save you from any random little issues or bugs.