hi David
implement core two swith on network for redundancy very good way but my advice you can use core switch provision asic switch for example 3500,5400
because you can vvrp configuration(virtual router redundancy protocol )with provision asic switch
if master core switch and default gateway down on network running quickly second switch active on netwrok for switching and routing operation
if you have now 2848 switches you can only switchign redundancy two core switch without routing
each edge switch must be connect with two uplink port each core switch
cenk