The GTX card cannot use your RAM, and it would not help in any case since it is
too slow - your DDR3 system RAM is not as fast as the GDDR5 in your GPU
and it will also add latency.
Nvidia supports SLI defined as :
Upgrade your rig with a second NVIDIA GeForce GTX GPU to enable SLI,
the system turbocharger of choice for gamers, which enables everyone’s
favorite titles to be played at the highest screen resolutions with
every setting maxed out. NVIDIA SLI supports the use of up to 4 GPUs,
and thanks to FCAT, SLI is certified by independent press as fast,
smooth and responsive.
SLI will work by sharing frames among the two GPUs, one rendering the odd frames while the other does the even frames.
However, buying a second GTX just for using SLI, with no guarantee that it
will really work on your computer, seems like a bad idea to me.
I would rather advise you to buy a new and better GPU.
The GTX 1070 is a wonderful card that will take on any and all video games.
Just verify first the power requirements of this beastie in view of your PSU,
as well as your available motherboard power connectors.