Or you could get a Xenon. Great server chips on Either X79, Z/H97/Q85, or wait for the E5 Xenon v3s on X99. Up to 20 cores (single chip that is, if you run Dual CPU, it's going to be 40 cores/80 Threads. Overkill I assure you, and way too expensive) , and are the best for servers and supercomputers. What exactly will you do on this server of yours? Consider looking in to Professional GPUs, 64 GBs of RAM and the 850 PRO SSD
Depending on application, but there is usually a minimum number of cores certain software will run properly on, and any more cores won't be as good for performance than GHz or IPC increase. Note that most consumer grade applications aren't optimized for more than 6 cores, but some pro applications can take 20 easily
Note: kunthakenthe said that core ixes are server CPUs, that is not correct, as the Core ixes are consumer CPUs and Enthusiast CPUs, and largely less stable than Core Xenons.