Firewire Performance Question...hardware limitation?

Hello All,
I've got a first Gen 24 inch 2.33 Ghz iMac maxed out with 3GB of RAM.
I started paying attention to how fast I can transfer about 2.5 Gigs of data to my Firewire 400 and Firewire 800 drives, as well as a local copy on my internal 500gb disk. I look at Activity Monitor when I do the copies. I'm not looking for a huge explanation, I just wanted to know if the numbers I'm seeing are expected for an iMac like mine that has a bus speed of 667.

Copy of 2.5gb file from internal disk to Firewire 400 drive
Result: 16 mb/s read (peak), 16 mb/s write (peak)

Copy of 2.5gb file from internal disk to Firewire 800 drive
Result: 31 mb/s read (peak), 31 mb/s write (peak)

Copy of 2.5gb file locally from one folder to another
Result: 41.5 mb/s read (peak) 41.5 mb/s write (peak)

Now I've seen some numbers out there where Firewire 800 transfer speeds beat my local file transfer speed. Is the performance I'm seeing expected for my hardware, and if I were to upgrade to a current 27 inch iMac, can anyone comment on the increase of Firewire & local disk performance I may see?

Thank you!