According to the SLIZone FAQ:
But I tried it anyways with my existing 7600GT and it seems to work.
Anyone else have an experience with this?
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.
But I tried it anyways with my existing 7600GT and it seems to work.
Anyone else have an experience with this?