https://bugs.freedesktop.org/show_bug.cgi?id=42373
--- Comment #43 from Jonathan Hamilton jtrhamilton@gmail.com --- I do not know if this is relevant, but I recently purchased a radeon 6450 and had similar issues - corruption when KMS is started and lockups when X starts. I did not get these issues in fglrx or the windows drivers.
I then happened to notice that the memory clock settings for the 'boot' power profile were incorrect, and were significantly higher than the card should support. The high & low power states appeared to be correct. Changing the boot values using the Radeon Bios editor (http://www.techpowerup.com/rbe/) fixed these problems for me.