The "PCX GPUs" are older than the first generation of SLI-capable GPUs. The GeForce 6 Series were meant to be PCI-E from the beginng, thus SLI-capable.
Initially the GeForce 6800 Series were realeased for AGP, but those weren't "PCX GPUs".
There are no such thing as a "PCX GPU", it's just a name to separate those designed for PCI-E from those designed for AGP. The PCX series carried a special chip (the HSI Bridge chip) in the PCB that translated AGP data to PCI-E data and vice-versa.