About the only thing I see as a possibillity at this point is the VIA chipset - Do you have the recent / the latest (4.38(2)) drivers for your respective chipsets (
www.viatech.com)?
Something else that might be worth trying, to try to isolate the problem to some subsystem, would be for you to get Qcheck (
www.netiq.com). It specifically tests throughput for the LAN by generating the traffic from RAM. Perhaps that will help to isolate the problem to one machine or the other, and whether it's a drive/storage issue or a LAN-related issue.
Maybe running some other benchmark utility (like Sisoft Sandra) that tests the computer in subsystems may help to isolate the problem subsystem.
Aside from that, and I'm suggesting this knowing that you've probably looked/done this, but we're in "stretch" mode here: Check the BIOS setting on both computers. Though I don't remember the specific chipsets, some will automatically rest to "safe" defaults when some problem is detected (and my recollection is that it can be a trivial problem). SO, perhaps your processors are underclocking, your RAM values are cranked to some slow values, and your drives may be set to PIO mode (any/all may severely impact your performance).
How is the cooling in your cases? Any possibilities that the motherboard(s) is/are detecting an overheat condition and throttling back the procesors? I'm not that familiar with Athlon chipsets but most Intel processors/chipsets will reduce clockspeed when an overheat condition is detected.
Do you have an firewall software installed on either machine? We've had a couple posts in the past like this where everything looks perfectly fine, but still have a problem, it turned out to be ZoneAlarm (or some other firewall software) still running in the background, even though the "front end" was shut down. If you have firwewall software installed, please remove it, just for testing purposes.
If you have some software or feature on your motherboard that allows monitoring the power system of your computers, check it out to see that none of the values (like the 12 volt rail) aren't running a little low (I think 5% is the usual tolerance level...I don't recall...but I think it's 5%). It's not likely, but is possible, that the power supply is a little light or weak and running at marginal levels, which may slow things down without causing instability. Like I said, we're in stretch mode here....
That's about it for me.
Good Luck
Scott