Wifi antenna booster?

Hi!
I live in Capital Federal, Buenos Aires, Argentina, on a 7th floor and with my Mac Mini Intel and Kismac in OS X Tiger I can see up to 7 wireless networks. The thing is that at least 2 of them are not protected in any way but I get such a crappy signal ( Kismac reports a max of 45 of 255 ), than i can't stay connected for a long time, and the speed is really really poor ( more than one minuto just to load google.com !!! ).
Anyways, I was thinking of buying an USB Wifi Antenna Booster but I really didn't know where or what to look for... I know that I need a great signal boost, and for it to be compatible with either Tiger or Leopard ( I can do the upgrade if need be ).

So, any recommendations?

Thanks!!!

Hey, just found this, not used any of their products but they may have a solution for you!

http://www.hawkingtech.com/products/index.php?CatID=35&AppleComp=1

For hours of fun, google this:
cantenna
I see you are posting from Argentina, the English word is a combination of "can" and "antenna". Here is one of the more famous ones:
http://seattlewireless.net/PringlesCantenna

Funny you should ask, I just saw this article today:

http://crave.cnet.com/8301-1_105-9882287-1.html

It is can boost the distance, I'm sure reception nearby would naturally be stronger as well.

I use a Linksys Range Expander as part of my wifi system. I would presume that it would work together with Airport.

I'd check the laws where you live. In the United States what you're trying to do is technically illegal