Have you got any other wireless stuff in the house? mobile etc? try turning all of those off, probably barking up the wrong tree but who knows..
Can you ping the router? what sort of response time are you getting?
As has been said try upgrading the firmware on the router and the cards, and drivers if available.
Have a look
here there are some good forums etc
[Edited on 22-11-2004 by jpmercer]