Get iStumbler or MacStumbler to monitor your wireless connection. It will also tell you if there are other WAPs nearby and what channels those other WAPs are using. You can also use stumbler software to monitor signal strength as you change the channel on your own WAP. Try setting your WAP to channel 6 or 9 or 11 and see what that does to your stumbler strength.
The airport signal bars mean very little, since Apple changed the software to measure the "quality of signal". Now, an iBook (much better airport reception) getting 4 bars will be much "better connected" than a PB also getting 4 bars of signal (to restate, bars mean zilch.) I had a friend's 14" iBook sitting next to my 12" PB, and iStumbler on both showed clearly that the iBook had better wireless reception. Previous airport/system software gave a more objective "wireless bar strength" measurement, where iBooks would show 4 bars and PBs would show 3 bars from the same location. Apple decided to gloss over the aluminum handicap by cosmetically changing what the bars meant.
In practice, internet speed varies widely from minute to minute. Google for "bandwidth test" and then bookmark that site: you'll notice speeds changing dramatically depending on time of day, etc.
Also, reception quality varies widely between different brands of WAPs. My Buffalo Airstation was previously top-rated from reception, but the Hawking I just got is consistently and significantly stronger sitting right next to it.