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The 9500 GT is going to slow down your performance. It just doesn't have the power to keep up with a GTX 780.
To give you an idea, Tom's Hardware did a dedicated PhysX card review in 2009. They tested several PhysX cards paired with an old GTX 260 graphics card. The 9500 GT was basically no better than no PhysX card. With a GTX 780, many times more powerful than a GTX 260, the 9500 will slow it down, add heat/noise, and basically not be worth it at all.
http://www.tomshardware.com/reviews/batman-arkham-asylum,2465-8.html
Maaan im soo much confused after ur post, the benchmark has "No Dedicated Physix" the lowest factor
and 9500GT option above it obviously saying its BETTER THAN NOTHING
I was convinced NOT to do it with first posts of our friends here and u're emphasizing on it too BUT isn't this picture here saying the otherwise?
dedicated 9500 has more FPS than No Dedicated Physix from what im seeing (thanks for the pic though)
and 9500GT is silent, at least more quiet than 780
im not gonna SLI them, they will be separate, STILL will they affect each other?