As long as your open server is just a firewall (i.e. distributed - the SMS is separate), 4GB RAM should be OK with so few blades enabled, 8GB preferred. If it is standalone (i.e. self-managed) I'd strongly recommend at least 16GB of RAM, however management operations will be very very slow with only two cores and some advanced management features will not be available. I can pretty much guarantee you will not be happy with the performance of the management tools like SmartConsole in that case, even though you are technically meeting the minimum hardware requirements for R80.10.
As long as the HCL says R80 is supported, R80.10+ should be too.
The Dynamic Dispatcher will be enabled by default in R80.10 so the heavy traffic flows should get spread out fairly well.
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