Just to clarify:
sport404 is right, CAT5 is designed for gigabit. CAT5E is designed for longer distances and high(er) interference situations.
CAT6 is designed for 10GigE. CAT6E has the same characteristic as 5E.
For 6-100 feet in an average home where there is no significant radio/electrical/magnetic interference, the "E" variants of ethernet cabling don't bring anything more to the table.
Another factor could be a substandard quality cable.
In terms of cable choice, get the best quality for the price no matter what the cable type is as long as it is at least CAT5.
Having said that- I too have a DIR-655 as well as the DNS-323 - WinXP, Ubuntu Linux, and Win7 - Raid-1 (mirroring). I'm getting the same speeds as you. There have been occasions where I've gotten 20-24MB/sec -- getting more would require RAID-0 or (more likely) a faster CPU in the DNS-323.