Gigabit Performance Problems

I recently upgraded my home LAN with an SMC8508T router and Intel Pro1000
NIC's. My setup is a mixed environment with a Debian 3.1 dedicated file
server, one WXPP offering a share, and clients running Vector Linux as well
as W2KP. They all communicate using SMB over Cat5e.

Using default settings, WXPP to W2KP saw an immediate speed boost, getting
around 28 MB/s. The Linux machines did not fare as well. Debian managed
about 7, Vector 5.0 did 9, and Vector 5.1 went to 20 or so. I tried some
others like Blag30K2, Knoppix, Ubuntu. Again, results were from single
digits to low 20's. I used the Live CD's on the Win machines to make sure
the cables were not a factor.

I experimented with tweaks using sysctl but results remained constant.

Suggestions welcome.
Roy