I can verify that a D-Link DI-604 works well enough with Second Life.. I use one.
Just as a random note, I have "Gaming Mode" disabled.
Its help says "Gaming Mode
If you are experiencing difficulties when playing online games or even certain applications that use voice data, you may need to enable Gaming Mode for these applications to work correctly. When not playing games or using these voice applications, it is recommended that Gaming Mode is disabled."
I'm not even sure what it does, exactly, or why it's recommended to be disabled when not playing them.
I have it set to 10/100Mbps Auto (Tools, Misc.), versus 10Mbps/Full which I tried once. Even though my internet connection is less than 1.5 Mbps, it all seemed less laggy with multiple computers when set on 10/100Mbps Auto.
I do have a 100Mbps network card.
And.. under the filters, servers, and all, I didn't have to set up anything for Second Life.
You might check what others are doing with the connection, as others said.. but a lack of bandwidth shouldn't be causing a low framerate/fps. It would just make things load more slowly and potentially delays in moving, chatting, building or otherwise manipulating objects in the world.
For me, at least, I know others loading webpages or downloading can lag me in SL. This router needs some traffic shaping, or something.

Though I am on a wireless internet connection, which surely contributes in some way.
While an area's loading, it will generally use the bandwidth I've specified, and after an area is loaded, it hovers somewhere around 6-20kbps.