I think I found part of the answer....http://www.slizone.com/page/slizone_faq.html#s4
"Can I add a 3rd graphics card in my 2-way SLI enabled PC to connect more monitors?
Yes, you can add an additional graphics card to your PC to connect two additional monitors. The additional graphics card must have a GPU that is different from your SLI GPUs and must be NVIDIA PhysX capable (GeForce 8 series and higher with at least 256MB of memory). For example, two GeForce GTX 2XX GPUs in SLI and a GeForce 9XXX GT is a supported configuration."
>>thing is, does that mean a GTS 450 is the same or a different GPU as the existing GTX 460 SE...what defines the GPU?