The latest Arnold 4.2.13.0 raises the minimum CPU requirements from SSE3 to SSE4.2:

• CPUs need to support the SSE4.2 instruction set

From the Arnold Release Notes:

SSE4.2 optimizations: We have raised our minimum CPU requirements from CPUs needing to support the SSSE3 instruction set to SSE4.2. This allows us to start taking advantage of the additional SSE4 instructions to further optimize Arnold. Already we are seeing a slight speedup in several areas. For instance, the Hosek Sky shader is now 5% faster.

If you want to check whether an older machine supports SSE4.2, here’s a few suggestions:

• Google your CPU and “SSE”
• Windows: Download and run coreinfo -f
• OSX: Run sysctl -a | grep machdep.cpu
• Linux: Check /proc/cpuinfo