'I have no airport card. But i want to see my signal to / from my neighbour.', I presume you mean - you want to access, and be accessed by, your neighbors computer wirelessly, via your ethernet cable connected Mac.
Yet, you did not provide the model of you Mac, the System / MacOS X installed on it; nor, the manufacturer, operating system, or model of your neighbors computer; as well as, the 802.11 b or g card's manufacturer and model number (if a PC), and the distance between the LinkSys router and neighbor's computer, and the number or type of obstructions (walls, fences, roofs, trees, buildings, vehicles, etc.) between the two.
'I can not find any tool that works without airport card ???', what tools are you referring to? Did you look at the WRT54G manual (also available
here)?
'Can anyone help ?', maybe so - but one cannot expect such if no (or very little) information is provided.