The intended role of the Nexus 5000 platform is high density access layer. The Nexus 5000 series does not have any layer 3 capability, or if you are speaking of the Nexus 5500 series, only has layer 3 when you add a special layer 3 card/module. If you install a layer 3 card into a 5500, it reduces the overall capability of the device by a significant amount. Generally you would use a Nexus 7000 or a 6500 to serve as the data center core and use the nexus 5000s to perform high density layer 2 switching at the data center access layer with the use of FEXs.
The data center core should be connected into the campus core using point to point routed links and all VLANs should terminate at the data center core devices. If you have a need for greater scalability, you could introduce an aggregation/distribution layer within the data center. If you do your layer 2/3 boundary should be on the aggregation equipment unless you want to go fabric path.
