Heat sink mystery.

Solution
If you can install the GPU in the right position, I mean the GPU can stand in the vertical postion and completely into the pcie slot, you are fine even the GPU touch the heatsink. But if you can't install the GPU in, that will be the problem. And you will need replace the heatsink with aftermarket heatsink, like this http://www.ebay.com/itm/38-x-38-x-10mm-Aluminium-Black-Heat-Sink-Heatsink-for-Northbridge-Southbridge-/252621866777?hash=item3ad16fc719:g:29QAAOSwrklVCGdA

You may try other similar gpu to see what happens, if you don't buy the gtx950 yet. Keep in mind the southbridge runs pretty cool, it is for controlling USB, pci, etc. https://en.wikipedia.org/wiki/Southbridge_%28computing%29
If you can install the GPU in the right position, I mean the GPU can stand in the vertical postion and completely into the pcie slot, you are fine even the GPU touch the heatsink. But if you can't install the GPU in, that will be the problem. And you will need replace the heatsink with aftermarket heatsink, like this http://www.ebay.com/itm/38-x-38-x-10mm-Aluminium-Black-Heat-Sink-Heatsink-for-Northbridge-Southbridge-/252621866777?hash=item3ad16fc719:g:29QAAOSwrklVCGdA

You may try other similar gpu to see what happens, if you don't buy the gtx950 yet. Keep in mind the southbridge runs pretty cool, it is for controlling USB, pci, etc. https://en.wikipedia.org/wiki/Southbridge_%28computing%29
 
Solution