I'm experiencing the same problem. I've tried different firmware configuration settings and I am only getting 8MB/sec between my PC and my NAS. I've even tried transferring between PC to PC and it's the same data rate. I've been transferring 1GB files and only getting 8MB/sec speed.
My setup:
Comcast cable modem --> DIR-825 Rev. A Firmware 1.0 --> Cat6 cables --> PC and WD MyBookWorld NAS
The PC specs:
Windows Vista SP1 with the network packet tweak.
EVGA 680i motherboard with 10/100/100
If I hadn't bought this router from a guy on Craigslist, I would take it back to the store to get a Netgear or a Linksys.

BTW, this router emits too much heat!! It is hot as
hell to the touch.