Today I learned that the "s" in SX means "single" and "d" in DX means double. The DX has double the data bus width of the SX (32-bit vs 16-bit).
Intel zapped the FPU on the SX to disable it. Apparently it cost more to make, but sold for less.
After the exchange of MMX and 3d-NOW, it was AMD that adopted Intel SSE into amd64.