I have a G31M-ES2L with Quad 8400 CPU @ 2.66 GHz and Nvidia GeForce 9600gt, 512 MB. Power supply - 400W.
Is there any chance for adding a second video card on this motherboard? And making them work together for some performance boost in gaming? I was told buying a better video card won't do due to limitations imposed by motherboard (and that current setup does not use full power of 9600's)
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That's a small form motherboard, only has one PCI-E slot, so no you can't use 2 video cards working together. You can add a second video card, but that's only for more monitor support.
For your power supply and CPU, something like a Radeon 6670 or a 6770 will work well, as long as the power supply is a good brand. If it's a cheap one, stick with the 6670. Either of those cards is around a $100 give or take, and both will be quite a bit faster than your 9600gt.
The only issue you may run into with using a 1.1 rev PCIe is that some 2.1 pcie cards won't work with it.
With a cheaper 400 watt power supply, you may not want to get a 6770 although it's a much better card. There is a 40 watt difference between a 6670 and a 6770 which is a large enough difference to cause issues. If you can find some extra cash to upgrade the PSU, your CPU is good enough to make use of a 6770.