Also i noticed you said you are using WPA security mode, is the WPA mode set to WPA2 only?
This is required to get better speeds, I use a DIR-655 with a Intel 5100AGN and had the exact same problem, it was fixed by:
- Setting the router to Auto 20/40MHz
- WPA set to WPA2 only
- Setting the Intel 5100AGN to automatic 2.4GHz so it uses 40MHz