I few questions;
What brand and model wireless router and wireless network card are you using.
Are you using 802.11a, b or g?
If you're using A then well...A sucks man...it has very limited distances out of the box.
IF you've got B it's slower (max of 11mbps) but it will do the best distances (out of box).
G is the fastest and is compatible with B out of box so it can adjust itself to B speeds to get the distance you may need.
If you can try moving the router to a more central location in the house. Or as a last resort add a second access point to extend your signal.
http://www.konfabulator.com/
That link is to a widget utility that might be useful in watching your signal strength but the best bet is using the utilities that came with your hardware and netstumbler as mentioned earlier.
Let me know.