Don't listen to nazgulnarsil, xonar. They are probably undercover evangelists, read:
Astroturfers for ATI, the company probably responsibly for this whole BitCoin thing.. lol. Just kidding, but wanting to make you think about the enourmous waste of resources of this project and who is the real benefitor; at the moment at least it is the hardware companies, foremost Foxconn
( https://secure.wikimedia.org/wikipedia/en/wiki/Foxconn#Controversies ).
I have the Sapphire Xtreme 5850 since 3 days now with
Artefacts all over my Desktop, both
in Linux Fedora Core 15, with or without Compiz and
in Windows. I plugged the powercable of my high-end Enermax PSU and gave it a 15 minute rest,
still seeing artefacts,
a lot of them. I mean it's really
really ugly, I can't see anymore.
This is all it took to destroy my poor Sapphire (RIP)
Clock to 890 (core),1000 (mem)
Agression=11
Fan=60
Temps: 60°C
Let it run 24h on that settings.
The card is now longing for the trashbin. The damaged VRAMs probably will cause damage to my not-so-cheap-either 1155 board, too.
Underclocked the card now to 300 (mem) and overclocked exactly 10% to 797MhZ (core). This gives me 15% less hash rate return, however a 100% return on my $199 investment in the 5850.