When upgrading a Nexus 7000 to NX-OS version 5.2 (using more than 1 VDC) or to NX-OS v6+, Cisco claims the need to upgrade the system memory to 8Gb.
Note I have run on v5.2 using only 4Gb per SUP using 2 VDCs and it has worked just fine, but I should mention that the box was not under heavy load.
See how much memory your N7K has on a SUP by using the following command:
N7K# show system resources Load average: 1 minute: 0.47 5 minutes: 0.24 15 minutes: 0.15 Processes : 959 total, 1 running CPU states : 3.0% user, 3.5% kernel, 93.5% idle Memory usage: 4115776K total, 2793428K used, 1322348K free
The upgrade per SUP would need the Cisco Bundle upgrade package (Product code: N7K-SUP1-8GBUPG=). One package has one 4Gb module. (see picture below) If you have two SUPs you would need two bundles. Notice the 8Gb sticker on module in the red block.