I have 64-bit 12.10 Ubuntu and I just put in a new SAPPHIRE 100354XTL Radeon HD 7870 video card.
Was wondering where to get suitable drivers. fglrx isn't working. Will the legacy drivers work with this version of the video card?
My experience trying to install/tweak a HD7770 recently (using xubuntu 12.10):
Apparently there is a magic script (smxi.org/sgfxi) that can fix things, but by the time I tried it my system was so broken I couldn't even get to a Ctrl-Alt-1 console to run it properly. In the end I gave up, reinstalled and bought an nVidia GPU and have found it much better.
As I said, just my experience. Hopefully you have better luck than me. I'd try Catalyst at least and see how it goes.