2.4GHz WLANs can experience interference from cordless phones, microwaves, and other WLANs; the building materials in your house could degrade the overall throughput. every environment is different!
What wireless adapter are you using? Also, which firmware? the latest on www.support.dlink.com is 1.01 (just double checking)
As frequency increases, range generally decreases. As a result, 5GHz systems generally have less range than ones operating in the 2.4GHz band. 2.4Ghz signals easily bounce off of most materials therefor increasing range. Whereas the 5Ghz band is very large signal by nature and has trouble reflecting off of surfaces however, it has MUCH higher penetrability in close range, but lower overall range.