I am using Windows 7 64 bit and have a Sapphire 5870 and have been experiencing regular crashes.
Sometimes its on start up loading the windows desktop screen, sometimes during normal use or playing a game.
Windows tells me there is a driver problem, but as it works most of the time and it has the latest driver installed
I am going to leave it as it is and wait for new drivers to be released.
I have read quite a few posts about Windows 7 and 5800 driver problems so I would assume it's a known issue.
I know this doesn't help you much, but if you are also on Windows 7 and you can use it with the default windows drivers
you too might have wait for a new release.