I have been looking for a way to add an antenna to a WPC54GS for a while now. I got it cheap (£3!) and it has a well supported Broadcom chipset. I have had good results with it under Backtrack 3, even if the TX power is only 15mW.
Here is a high res pic of the board: http://www.bagpipes.net/files/front-linksys54g.jpg
Sorry for being a n00b but where do I need to solder on to? I found an old thread (http://www.netstumbler.org/f9/wpc54g-version2-external-antenna-w-tnc-jack-14698/) which says connect the pin to the end of C155 (top left corner of the image) but checking with a multimeter this appears to be connected to the GND points nearby. Maybe my multimeter isn't good enough to spot the very low value capacitor in between? In that case presumably I should just solder shield to one of these ground points...