I added a GT 640 to my computer today for PhysX processing. All went well thru the post but when Windows should have loaded I got a blank screen. So I removed the newly added GT 640 and my computer is working normally. My guess is the PSU is underpowered. I ordered a 1000 WAT PSU tonight but am asking for second opinions if my PSU is underpowered prior to installing a new one.
Well, without fans or USB devices, it came out to 535W (without overclocking that is). You probably know your computer better than I, so you might want to check the calculator for yourself.
I still can not get the GT 640 to work with my GTX 680. I did get a new 950 WATT power supply. http://www.newegg.com/Product/Product.aspx?Item=N82E168... . The power supply install went well, computer boots thru bios screen okay then blank screen. It does not matter which DVI port I use, the BIOS will show and when the splash screen should appear I get a blank screen. To clarify, the screen is not intirely blank. Ther is the smallest sliver of color (1 pixel by 10 pixes horizontal) in the upper right screen corner. When the screen blanks the HDD light also stops.
Try outputting from whichever graphics card you're not using, and, if it works, re-enter BIOS and (I believe you can) select which one has output priority.
Okay my BIOS does have an option to select one video card as primary and then the second card shows non boot. To clarify the only options per each card is primary or non boot. If I set one to primary then the other automatically sets to non boot. I tried both the 680 and 640 as the primary boot and each will work thru the BIOS screen then blank when Windows splash screen should appear. My video cards are: PNY VCGGTX680XPB-CG GeForce GTX 680 and PNY VCGGT640XPB GeForce GT 640 1GB. Thanks for the help so far from The Indomitable.