lilrags16:
hello, i am building a automated greenhouse controller. the Ardiouno uno needs to receive commands from the network( via Ethernet card) and save them to the SD card. The SD card should be read for the variables( temp, humidity, watering times, light times). it will output to the free pins data to a more advanced MC for controlling relays. Please help!
You say "building" but I think you should stop immediately, sit down, take a deep breath, do some sensible research, and start over again - with a different concept.
For starters, an Arduino is quite capable of running the relays. You may need a Mega but that is merely mega, not more advanced.
While using an SD card may be a good idea, it need only be used to record what Arduino did. What you are suggesting is that it be used to tell Arduino what to do, and I believe there are simpler, cheaper and easier ways to do that.
You probably don't need an ethernet. You might need one if you cannot install the Arduino in the greenhouse. You might need one if you want it to communicate with the outside world, but that is not what you are suggesting.