I think I have the same problem like you!
My problem occured after a world update on my gentoo box. (testing version with kde 3.5.5)
I also have a geforce 6600 gt and I have testet the newest beta and stable driver.
In the console my computer doesn't freeze.
When I boot W*****S my machine works very well.
I think I found an another thread about this problem:
I don't know what I should test now!
By the way: I have succesfully used the beta driver with beryl, so then I have disabled it, but the error still occur.