Large HD is not necessary or useful. In fact, it is generally counterproductive.
Concept: You should not be storing large quantities of Data on your Vicidial server. Recordings, logs, etc should be deleted or moved to an archive server as soon as possible. The Vicidial server "overworks" hard drives, and they die. When they die, everything on them is gone. Unless you have moved them elsewhere (or unless you have large RAID drives to avoid loss in case of drive failure).
Additionally, large HDs are generally SLOW. Slow is counterproductive. Your best bet is a RAID10, 5 or 6 drive SAS15K system. RAID10 keeps the speed viable while reducing the likelihood of loss of data. More drives increases speed and allows the system some resilience in case of drive death. More drives means more duplication of data, which means the system can continue at full speed after loss of a single drive (and you can repair the RAID after hours, without down time for RAID rebuild and without "limping" during the shift.
Specific processors are inconsequential. Number of cores and processor speed combined with a chipset compatible with the installation .iso .. those things are quite important.
Be sure the Motherboard has been "on the market" for over a year. This is generally an indicator that the chipset has drivers available in most linux distros by now.