You will not be able to do much if any overclocking to a Xeon processor. They have locked multipliers which means you can only overclock by the BCLK which overclocks everything and is much less stable.
And since that is a socket 771 chip, no processor in that socket will be fast enough for a 970. The oldest I would personally go with a 970 is Z77 motherboards. 1155 is still fairly decent for gaming unless you have a 1080 or a Titan X 2016 which then I believe there is a bottleneck.