I actually was running on a SB5100 until last week, I was on the blast plan and it always maintained 35+Mb/s speeds with that old 2.0 modem. My neighborhood is fairly quiet with mostly older families so I never noticed any big drops in speed during "busy" hours.
After the latest speed bump though, I decided to run out to that 'yellow tag place' and grab a SB6141 they had on sale for $90. My results after the upgrade are a decent bump in speedtest.net speeds, but honestly I don't notice it much while browsing around. Downloading large files I certainly do notice a difference though, it's roughly twice as fast as it was before.
Looks like I'm in the same area as you, here's one of my better speedtest.net results: