External Antenna vs. Airport Extreme or "repeater"

Hello everyone and Merry Christmas - Happy New Year - etc etc!
I have always had a problem with my Airport Extreme (located in the back of the house) signal reaching my iMac G4 in the living room in the front of the house about (all of) 30 feet away. Sometimes I get a fair signal of 2, maybe 3 bars but often get one or perhaps a fractions of one. It's tarpit city. I presume that there is some interference in the house, since I've tried every channel and there seems to be nothing to choose among them.
I'm considering buying an external antenna for the Airport to boost the signal, but wonder if just getting an Airport Express to extend the signal might do the same job.
I'd like some feedback from others who may have tried either or both options.
* Does an antenna really improve the strength of the signal from the Airport?
* Wouldn't the Airport Express just encounter the same interference and therefore be ineffective for what I need?
Many thanks to all - I haven't been here in a while - the website looks good, Ric!
-adriana