Can only boot to safe mode since installing OS X update 10.9.3

I now realise that everything was working fine on my iMac up until the point I installed this update which I now note has graphics updates.

Bascially if I do not boot in 'safe mode' then I am unable to log on to the machine. Instead of getting the logon page where you can see the users, I get a gray screen with corrupted/chopped up user names and from here can't do anything (so it gets past the apple logo, tone and so on)

How can I fix this?

I have tried all the steps shown in http://lifehacker.com/how-to-troubleshoot-the-four-most-common-oh-sh-t-mac-48708 0511

and still I am only able to boot into safe mode

iMac 27" late 2009

12 GB 1067 MHz DDR3

ATI Radeon HD 4850 512MB

OS X 10.9.3 (13D65)