There are many things that can account for a weak signal.

1. Distance from AP
2. Other technology such as Cell Phones, Cordless Phones (check if the cordless is the same freq of the AP i.e. 2.4Ghz), Microwaves, etc.
3. The thickness and amount of wall material between AP and client
4. Not the right driver for the wirless card. Since this is a Unix forum I would check out ndiswrapper if you haven't already.

afaik linksys has speed boost not a range booster. But another option is to get a dual band linksys (or like) for greater range.