there are many wireless card (Internal card or USB) for the desktop... go for linksys.
As for mac (are you sure it doesn't have one?).. I don't know any other brand than mac/apple wireless card, ask your mac shop...
It should directly get the signal, after you enter the SSID and password (you should have... for security reason, also use mac filtering and enabling wep/wpa/wpa2)
I don't know about your salesman said .. but normally you should be able to connect using 1 broadband.
I recently connected a laptop notebook to a cable modem by way of a internal wireless connector and through a linksys wireless router to attain a internet session.
There was no apparent problem.
However, I now need to apply the same process to a dell dimension desktop and a Mac G4 computer so that
they too can use the broadband cable service.
1.)Can you recommend a good internal wireless connector for a Mac and a Dell desktop?
2.) I am using optimum online which I was told by their salesman that I need two installation kits (1 for each computer)? Is there a better way to do this?
3.) Can I also use the Linksys wireless router, currently being used with the laptop notebook, to send and recieve from these desktop computers?
Thank you ahead of time for any help regarding this issue.

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