There are a few differences between i7 and Xeons but none to put I7 in front of xeons.
Xeons are usually made for 24/7 operation as you stated but I7 could allso provide that.
Main advantages of Xeons over I7 is ECC memory support as well as larger ammount of memory supported. Technologies are also better on Xeon side when working with large ammount of data, see Demand Base Switching.
I also do these kinds of surveillance workstations and always go for Xeons as they offer better reliability and better efficiency. Please note most I7 have a higher power consumption then Xeons.
For surveillance onboard videocards do just fine, stuff like : XGI, Matrox etc.