Heavy upload server scalability


We are running a backup solution that uploads every night the files our clients worked on during the day (Cabonite-like).
We have currently about 10GB of data per night, via http PUT requests (1 per file), and the files are written as-is on a NAS.
Our architecture is basically compound of a load balancer (hardware, sticky sessions), 5 servers (Tomcat under RHEL4/5, ) and a NAS (nfs 3).

Since our number of clients is rising, (as is our system load) how would you recommend we could scale our infrastructure? hardware and software? Should we go towards NAS sharding, more servers, NIO on tomcat...?

Thanks for your inputs!