Server multi-tasking failure

[therealnerd2]

Hi all,

I have a server running:
Windows Server 2003
Opteron 144
1x 200GB OS Drive on onboard SATA controller.
4x 500GB Data Drives in RAID 5, using Adaptec 2410sa on regular PCI bus.

Its linked to my main machine (E2200) with a gigabit lan switch.

Here is an example of the problem:

Copying files from main machine to OS drive on server:
CPU usage: ~30% - LAN usage: ~40%, so it would seem to be limited by the hard drive write speed at around 30mBps, fine.

Now try to do any file system accesses on the data array (even sat at the actual server) it completely fails. I cant even watch a video needing 160kBps.

CPU and LAN usage all seem low-medium.

My only theory is that the data transfer is maxing out the PCI bus, and preventing any other data getting through. EDIT: Having read max 32bit PCI throughput is 133MBps, I am totally stumped.

Note this is only an example of the problem - the same thing occurs in most other configurations - writing to raid reading from OS, etc etc

Any ideas how to sort the thing out?
Thanks,
Sam