Some folks over at the folding forums are reporting issues with their 450s and the 6811 WUs. Too early yet to determine anything for certain, but I would certainly take the advmethods flag off if you have a 450 (but finish the WU if you can).I had that problem for a bit but I'm not sure which of the changes I made actually fixed it, sometimes I'm too impatient...
It was one or all of the following;
Upgrading to the 6.40r1 client.
Removing the " -forcegpu nvidia_fermi" flag.
Setting the 2d clocks to be the same as the 3d in Afterburner.
One of the above also cured the downclocking issue I was having...