Firstly, which revision is the D-link and how did you get the driver since Rev B's is not in the 2.6.38 kernel natively?
rev A: dev 3c10, driver in kernel for ath5k/ath9k; see
http://wireless.kernel.org/en/users/Drivers/ar9170 ; firmware required - the driver page shows that 802.11 a/b/g are working, which suggests the driver doesn't have n-capability yet (and probably never will since it's deprecated).
rev B: dev 3c11, Driver available from manufacturer:
http://www.ralinktech.com/support.php?s=2 , in kernel since 2.6.29; firmware required.
You need to give us more information on what you are doing. Are you plugging these dongles into your computer one at a time, or both into two separate computers? Is your reference to win 7 a reference to dual booting or to another machine?
If both dongles are in use at the same time, The Chief's comment might be relevant since the D-link might be unable to use n mode in Linux.