I have recently bought a GTX650ti Boost, and replaced my GTX650 which wasn't quite cutting the mustard if you know what I mean.

Today, I decided to put the original GTX650 in my 2nd PCI-E slot, and dedicate that card for PhysX on games such as Metro. When I tried to run Skyrim, the main screen shows, but when I continue a game, it seems to load for a few seconds then I get a CTD. It was working fine just a few hours before I put the 2nd card in.

Any ideas?

Yes, don't use the second card. Skyrim does not like multi card GPU's on most systems anyway.

Not sure if you can use a different chipset like this, I know that SLI requires the same chipsets on both cards. Since the game can not take advantage of the Physixs processing the way that you are trying, perhaps it is trying to run the game under SLI.

Cheers, will take out the 2nd card and keep it as a reserve.

Actually your problem is you try to operate two different cards in SLI mode !!!!!

No, you cannot SLI. (not exactly, but not recommended)***

The SLI configuration is done in the manner when the model of the cards are same irrespective of the company you use for the card.
For SLI you have to have both the card capacity,core clock,cores same for SLIing.

The reasons why you cannot SLI (without having problems):
1) Both are different models.
2) The 650Ti Boost is having more cores then 650.***
3) Core-Clocks are different.
4) Even the capacity are different.

5) Different Memory Interface Width (that's you real problem)

*** GTX 650 has 384 CUDA Cores. and the GTX 650ti Boost has 768 CUDA Cores !!!!!!

Don't think that just because there is a TI at the end of one that they are essentially the same card, they're not.

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***Beside that all above, actually you can, but you will have only 1 GB of DDR5 memory instead of 2 GB DDR5.

(Actually, Vram and core clock can be different. Then both cards will run at the slower speed (for example: 2 cards at 1gb and 2gb will have a listed dedicated memory of 1gb).

or

you can try to put modded drivers, but your system can be very unstable...!!!!! (they are working nevertheless for the most cases !!!)

(It's probably best to do a fill driver sweep and then install the modded drivers. That way there isn't a conflict between the two.)

BE AWARE. YOU SHOULD PUT CARDS TOGETHER YOU WANT TO TRY IN SLI BEFORE YOU INSTALL MODDED DRIVER TO WINDOWS DETECT NEW HARDWARE.

http://anatolymik.itzod.ru/DifferentSLI/290.53/DriverWinVista7x64.zip
http://anatolymik.itzod.ru/DifferentSLI/296.10/DriverWinVista7x64.rar
http://anatolymik.itzod.ru/DifferentSLI/301.42/DriverWinVista7x64.rar
http://anatolymik.itzod.ru/DifferentSLI/310.70/DriverWinVista7x64.rar
http://anatolymik.itzod.ru/DifferentSLI/314.22/DriverWinVista7x64.rar
http://anatolymik.itzod.ru/DifferentSLI/320.18/DriverWinVista7x64.rar

To install:

1) Put cards together you want to SLI
2) Reboot
3) Download the modded driver
4) Log in as Administrator
5) UAC must be disabled
6) run Install.cmd
7) reboot

Video tutorial:

http://www.youtube.com/watch?v=XH46uLDxxHY

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LINK: http://www.techpowerup.com/forums/showthread.php?t=158907