340.52 gives some people problems (link) just like every driver has/does..You should try uninstalling the driver, not updating it..Perform a Custom (Advanced) clean installation (link) choosing only to install PhysX along with the Graphics Driver, enable SLI and see how it goes..You can install any of the other optional driver components anytime if needed/wanted..Always a good idea to restart even if not prompted to do so after installation..Alternatively, you can try Display Driver Uninstaller (link) to uninstall the driver and leftovers before you install it/another driver (link) again..Choose the Clean and restart (Highly Recommended) DDU option.

Tried those suggestions... still nothing. Now instead of a black screen a single white underscore in the upper left hand corner sits there flashing.. never stops been at it 5 minutes now.

 
I had the issue early on.  It's Windows that is the issue.  After you install, Windows tries to install drivers as well.  Give this a shot.
 
 
 1) Flickering Screen/Black Screen After Installing Drivers on  Windows 8
 
 Issue  After installing the latest driver on Windows 8, the subsequent reboot results in a black or  flickering screen.
 
 Root Cause  This occurs because Windows Update performed a background installation of a Release  3xx driver, which requires a reboot to complete the installation. After installing the  Release 3xx driver and then rebooting the system, installation of both the Release 3xx as  well as the Release 3xx is completed, resulting in conflicting driver binaries and the black/  flickering screen.
 
 Workaround - Prevention  To avoid the issue, reboot the system before installing the Release 3xx driver.  Alternately, you can check the driver status under the Device Manager and perform the  reboot if the Device Manager indicates that a restart is needed. Then install the driver.
 
 Workaround - Recovery  If you did not perform the necessary reboot prior to installing the driver and now  encounter the black screen, do the following:  1 Reboot in Safe Mode.  2 Uninstall the driver.  3 Disconnect from the internet and then restrt the system.  4 In normal mode, install the new driver." - The Workaround - Recovery fixed my problem people. After all it appears some people had it too.

Try this...
 
#1 Create a system restore point. 
 
#2 Download & Install CCleaner & Driver Sweeper 3.2.0.
 
#3 Remove all NVIDIA software from add/remove programs.
 
#4 Reboot into safe mode with networking.
 
#5  Run driver sweeper 3.2.0, check mark NVIDIA - Display & NVIDIA - PhysX, select Analyse button, if a list of entries pop up select the Clean button, when asked to reboot select Cancel. Do the scan again to make sure all entries are removed/deleted, if all entries cannot be removed/deleted please post that the entries cannot be removed/deleted along with a screenshot showing what cannot be deleted. If the entries were removed/deleted successfully continue onto the next step...
 
#6  Run ccleaner, select the registry tab, select scan for issues button, select fix selected issues button, when asked to backup the registry select "no" (we made a restore point earlier, we don't need to worry about backing up the registry entries.), select fix all selected issues button. Do the scan again to make sure all entries are removed/deleted.
NOTE: Two entries may still appear after multiple cleanings (it should be one or two and no more), this is normal and should be ignored, all other entries should be gone. 
 
#7 Delete NVIDIA GUID's using regedit. Click the start button and type regedit and hit enter. For windows 8 hit the windows key + r and type in regedit and hit enter. Navigate to...
- HKEY_LOCAL_MACHINE\SYSTEM\ControlSet002\Control\Video (highlight the Video key and press the delete key.)
- HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Video (highlight the Video key and press the delete key.)
NOTE: When deleting the video key you will be prompted with an error saying "Error deleting key, Cannot delete Video: Error while deleting key." Just ignore the error and select OK, all keys within the Video key will be deleted except for ones that cannot be deleted.
 
#8 Perform a clean boot of windows. 
- Instructions on how to perform a clean boot of windows can be found HERE.
 
#9 Uninstall Precision X/Afterburner, make sure to select "no" to saving profiles when asked.
 
#10 Reboot PC.
 
#11 Perform a clean installation of video drivers.

1. Download & save 337.50 driver to your desktop. 
2. Double click on 337.50-desktop-win8-win7-winvista-64bit-english-beta.exe located on your desktop & run the extractor.
3. Select custom advanced install, under custom installation options uncheck all options but graphics driver & physx system software, if you are using audio over HDMI please select the HD Audio driver, if you have a 3D capable monitor and would like to use 3D at some point select the 3D Vision driver also, check mark perform clean install, then click next to install the driver, reboot when asked.
4. Re-install Precision X/Afterburner, do not install both choose one or the other.

 
#12 Test and report back.

Sajin, tried all of your suggestions.. did not help at all.
 
I decided to format my win 7 after that and installed Win 8 Pro. I install 337.50 drivers and as soon as I went to enable SLi I got a BSOD. irql_not_less_or_equal and it reboots.. returns to the desktop with SLi disabled.

Clocks are set to default settings. Ram 1 stick at a time, still does it.
 
Passed twice on memtest without issue.

Have no other system to test in.. I don't even know anyone with a system that's SLi capable.

Okay...just so we know where we are at. Prior to trying to installing the 340.52 drivers you had a working system in SLI, right?
The only change you made between your working system and your current problem is installing the drivers, right?
For troubleshooting, you have basically formatted the drive and installed a clean copy of Win 8 and the problem is unresolved, right?
Lets step back a bit. Have you tried with a clean install of Windows 7 using the prior driver that worked? Lets try to ensure that you're not having a hardware issue and get back to basic functionality with a configuration and driver you know used to work.

Thanks for all the help guys, I learned a lot.
 
I ended up flashing the stock/default BIOS onto each card.. I never flashed a 3rd party BIOS onto either card, however I decided to give it a try anyway. Everything is working fine again and SLi is functioning properly. Just pulled an 18577 on Firestrike.
 
Now what could have happened with my Bios on either card? Do you think it corrupted on a card and they didn't match somehow so it wouldn't work? Just curious.. I'll have to keep an eye on my system closer for a while and see if I experience issues.

40Water
Thanks for all the help guys, I learned a lot.
 
I ended up flashing the stock/default BIOS onto each card.. I never flashed a 3rd party BIOS onto either card, however I decided to give it a try anyway. Everything is working fine again and SLi is functioning properly. Just pulled an 18577 on Firestrike.
 
Now what could have happened with my Bios on either card? Do you think it corrupted on a card and they didn't match somehow so it wouldn't work? Just curious.. I'll have to keep an eye on my system closer for a while and see if I experience issues.

Strange. A driver shouldn't ever corrupt your vBIOS.