It is pretty difficult to soup up the WiFi receiving ability of a PDA. Usually you crank up the WiFi signal on where it is being broadcast on. Are you connecting to your own WiFi network...ie a WiFi router connected to you DSL/Cable which is feeding your PCs/notebooks/PDAs? The 'cantenna' looks interesting as an add-on.
Basically you are going to be buying a souped-up broadcast antennae that hooks up to your WiFi router (or WiFi hub). I have never personally done much with this tech as usually my IT support team takes care of it. There are some antennaes (in-brand and third party) which basically are just an extension with some juice. There are other which server more as repeaters (I think).
All that being said ... on many of the soho WiFi routers/hubs/access points you can log in and change some settings which may increase signal strength but not by much (FCC and such are pretty stringent on signal broadcast). You also may be able to increase the signal strength on you PDA wifi adpater...but will vary with brand (not 100% on this though).
I've know a person who has used a 1 watt amplifier with a directional 14 db panel antenna to extend the range of a wi-fi connection about 2 blocks. He's using it to extend an office voip link.
For mobile use, there are some extended range PCMCIA cards that have high power (200mW) and external antenna connections: