Good.
RWIN is the receive buffer size. Windows tends to set this too low.
Your Tweak results would show you an optimal range for this setting, and the DRTCP freeware utility linked on the page could be downloaded to adjust the value.
But your results are indicative of an autonegotiation failure, or the use of the wrong cable, or a manual setting for 100/Full Duplex.
My guess is that the issue is the first one: your modem and the router (BEFSR41 has 10 mbs/Full fixed WAN port; except for Version #3 (relatively brand new) that can do both 10/100, and your autonegotiation is failing.
Purchase, dig up, borrow, an inexpensive switch. Plug modem into the switch. Plug the Linksys into the switch. The switch will handle power level differences, and most importantly, autonegotation logic between modem and the Linksys router to get you up to full speed.
Alternatives: Some have said with Motorola and Toshiba modems that you can take a 75' (foot) cable (excess length coiled) to connect the two devices and this resolves the issue without a switch.