I have been reporting a similar problem(?) for my 8700M GTs in my dell xps m1730 running stock Ubuntu 9.04 (with all updates). SLI has caused a complete lockup since the 185.x version of the drivers. 180.x and previous driver versions worked. 185.x and 190.x don't. My thread is here
. You will see I have become quite frustrated with the whole situation. I may as well be talking to myself :-/
In my case, I don't get to a log on screen. It crashes where I would expect to see the nvidia logo come up before the logon screen is presented.
It may help if you can run an nvidia-bug-report; even if it's not from the actual crash, it may help someone to help you.
PS: I haven't tried 190.40 yet, though the changelog/readme suggests there will be no change for this problem, ie it still won't work