Hey, neighbor !! I'm over here by Desert Sky Pavillion -- on Cox Cable @ 20/2 tier -- Like you, I have a Dir-655 behind an SB6120, but the D-Link feeds my Wife's DELL 3100, (P4@ 3.06 GHz, 2GB DDR2 Ram, Nvidia Graphics, 10,000 rpm Velociraptor,) with a D-Link DWA 552 ExtremeN PCI Adapter and a 3x5db Rosewill standalone antenna. Wifey rocks wirelessly at 25 down and 3.5 up most of the time.
My rig (DELL Vostro 230MT, Core 2 Duo @ 2.93 MHz., 4 GB DDR3 @1333, Zotac 1 GB 240GT Graphics, runs off the DIR-655's #1 port switch, with Cat 6 shielded Ethernet cables throughout.
I've found that the weather has a decided effect on the Modem's "Signal" as shown on the SB6120's stats. High outdoor temps (110+) tend to see a drop in downstream SNRs -- like from 37 to 35, for example.
Upstream power levels tend to rise with the hotter temps -- like from 46 t0 50, for example.
While both those numbers are within the "acceptable" range, they both represent a move AWAY from the "ideal".
On the SB6120, I find the optimums at a Download SNR of 38, and a Power Level at 0 ( all 4 channels)
Upload Power Level best at 45 --- with 55 readings, it's time to call the paramedics !!
I've also found (the hard way) that the DIR-655 and SB 6120 work best when they are at least 4 feet apart -- when placed near each other, they generate a bunch of interference that gets them acting all goofy.
Regarding your query -- Yes, the speeds do get to jumpin' around when the 'net traffic is high, but unless you're trying to do massive downloads during the evening, you should be good to go !!
I'm not a techie or a geek -- just an old freak that likes to see how well our low-budget machines can do against the Big Boys.
Good Luck !!