local computer access to NAS through USB port

No, none of the ReadyNAS can be used that way. The USB connections support peripherals (e.g. disks), but cannot be used to connect to another PC.

The readynas 104 have dual gigabit lan. You could connect the offline PC direct to the second port using a crossed wire.

No need for a crossover cable. However the 102 won't be able to saturate two gigabit links. Unless you are troubleshooting a performance issue I see little point to a direct network connection.

I think you'd really need to look at the 516 for the two NICs to provide a performance benefit.

The idea is to maintain network isolation for one computer, not to increase performance.

The RN102 doesn't have 2 nics, but the RN104 and RN312 do. You could directly connect a computer to the second nic, and assign static IP addresses in a different address range from the main network. As mdgm says, you don't need a crossover cable, because the NAS NICs autosense.

However, I am not sure this setup fully meets the requirement for network isolation. If the goal here is to ensure that the computer cannot be hacked, it might not be enough - since the NAS could be compromised, and then used to reach the computer.