If the aerial for the wifi card is stuck at the back of the Mac case, and possibly also behind your screen, then that could be a problem.
Also, have you got a lot of other wifi networks in your area?
My solution to the former was to buy an aerial extension (Maplin, £9.99). I also added a home-made parabolic directional surround (think half a tin can here
) to help cut out extraneous signals.
For the latter, see if you can ask your neighbours what channels they are using - for b and g there are only 3 non-overlapping channels (1, 7 and 13 in the UK, IIRC - 1. 6 and 11 in the US). You should try a variety of channels to see which gives you the best signal.
It is also possible that there are sources of interference in your house - microwaves, cellphones, cordless phones etc. Try switching these off to see if this improves reception.
If all else fails (or even before!), you can try turning the aerial through 90 degrees, either on the Mac or the WAP.
SD