Well, you could if the Intel S5520SC motherboard supports overclocking. You'd probably need an aftermarket HSF as well. However, the main point of getting a Xeon is for stability and overclocking can cause instability. If you want to overclock and only need one processor, then get a Core i7 980X - unless you have demands which require more than 12 threads and 24GB.
I'm just wondering though, why get one Xeon X5650 and a DP board? Are you thinking of adding another one later on?