Networking :: Transfer Rate For Gigabit Ethernet

We have a Linux box which acts a a file server. Currently, files and directories are exported using NFS.At the moment, we are a bit concern on its data transfer performance. FYI, we are using a embedded Gigabit Ethernet port on the file server. We ran a few simple write tests between NFS client (also utilizes GigE port) and the NFS server. In these tests, both NFS server and client are both connected directly to each other with a Cat5E cable. Unfortunately, the write/transfer speed results are not as per our expectation. It scores roughly about 11-12MByte/s, where as theoretically Gigabit Ethernet transfer rate is able to reach up to approximately 120MByte/s.I wouldn't expect to reach the theoretical max transfer rate (it would be great if we can , but I would appreciate if you guys can share with us in terms of the following :

1) What's the practical max data transfer rate which you guys managed to observe in a normal Gigabit based connection? What about jumbo frames configuration?

2) Is there any additional tuning/configuration we need to do within the OS to reach those practical max data transfer rate figure?

3) Does PCI-e / system bus plays a role in achieving this speed? For example, we are using the embedded GigE port and we heard some people says embedded ports are actually sharing the system bus and resources with other devices, which might adds into performance issue. Correct me if I'm wrong.

4) Does converting to Cat6 cabling will guarantee an increase in the data transfer performance?

5) In the future (once we are clear on how much single GigE transfer rate we can go) , we are looking into doing bonding since that the NFS server's shared directory/volume read-write speed is way much higher (i.e 400-600MByte/s). Will bonding allow us to achieve higher NFS read/write speed? What are the bonding modes best used for this purposes? Appreciate if anybody who has experience in doing bonding for NFS can share their experience.