The oldest hardware supported by the ixgbe driver is the 82598, which supports up to 16 RSS queues (see Table 3-48 in the 82598 datasheet). I believe that the 82599 and X520 are more capable. I have no idea what the situation is on igb, as there seems to be a lot more hardware supported by that driver and I don't work with the 1GB devices very much.