I'm using a laptop with a mobility radeon hd 540v. I'd like to install the driver recommended by the AMD website. Is that fine or should I install one from the "additional drivers" menu? Do I have to deactivate anything first? I'm asking because I'd like to get the full performance out of my card.
the driver from the "additional drivers" menu is actually the AMD one. It could be that it is not the latest driver, but at least it's a driver known to work with your Ubuntu version. And also, installing it from the "additional drivers" menu is very easy, while installing the driver from the ATI website can be tiresome and tricky.
So, I would install it from the "additional drivers" menu. That's the propietary ATI driver and will give you 3D accelleration.
ps: if the menu shows the choice between two drivers (one normal one and one saying 'post-install updates'), do NOT install the post-install updates. That has been known to break systems in some cases and give lots of error messages.