Slow network speeds between Linux RAID box and Windows PC

I apologize that this question is not directly related to mdadm, but I
thought some of the list might have experience in a mixed
Linux/Windows environment.  I am running a mdadm RAID5 file server on
a Ubuntu 8.04 Linux box on a gigabit LAN.  I am using a Highpoint 2680
card at PCI-E 4x as the controller (I have also tried the motherboard
SATA connectors directly).  I am running SAMBA on this box to share my
RAID5 with my main Windows PC running Windows 7 64 bit (although I
have gotten similar results with Vista also).
My question is related to what I consider slow speeds when reading
from or writing to the array from my Windows box.  I get anywhere from
20-30MB/s in either direction when transferring a single large file (3
GB as an example).  I haven't done detailed benchmarking on the array,
but spotchecks of file transfers from another drive on the same linux
box are giving me speeds above 100MB/s for reads and writes.
Does anyone have any suggestions for improving my speeds across the
network?  I don't know if SAMBA is causing some kind of bottleneck or
if Windows is not playing nice or if it's something with my network
configuration, but I was hoping to get alot closer to saturating the
gigabit connection.  Any help would be greatly appreciated.
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