From my experience (with centrino laptops), getting a different card won't make a difference. Using the "high speed" G cards won't help, since their gimmic is geared towards file xfer speed between computers, not increasing/solidifying the connection itself (this is a white lie... for the Dlinks, there is an eXtended Range mode that I'm not a big fan of, since it never helped me in getting a better signal.)
Is your sister on the cordless phone a lot? Do you use those energy-saving spiral fluoresent bulbs? These interfere with the wireless signal, since both the phone and the wireless router run on the same frequencies, and the bulbs interfere a little too. You might look into getting a higher gain antenna for your router, or purchasing a repeater if your signal woes continue.
A more extreme approach would be to invest in one of the newer MIMO routers and the MIMO cards - these offer a wider wireless range, without the interference of 2.4GHz devices.
Hope this helps.