It sounds like the Mac may be too far away from the linksys. They should be compatible. Make sure the Airport software is as up-to-date as possible. On Jaguar you will not be able to run the latest Airport software, as that requiers Panther.
By the way, if I may offer further insight into this. Both the Linksys WAP device you have and the Airport running on Jaguar will only support 128 bit WEP as the maximum encryption. As I work with security, I personally do not find this exceptable, because of inherent weaknesses in WEP. It will obviously have cost associated with this recomendation, so you will have to determine if a secure network is worth the cost or not.
I would recommend buying a wireless access point that supports WPA. Then configure the WAP device to use WPA for security, run as a closed networked, and allow only the MAC (Media Access Control) address of the Airport on the Macintosh to join the wireless network. However to use WPA security on the Macintosh, you will need to update the Airport software to at least version 3.3, and to do this you will have to be running Panther. As I said there would be a cost involved, but for me, I would do whatever I could to stop someone from accessing my internal network.
[Edit]I hate giving out bad info, so I had time to search more this morning, last night I went to linksys site and the sales pitch still only says WEP, but this morning eent to linksys site to verify info and searched further, Linksys says they have added support for WPA, so you may be able to flash the firmware on your linksys if it does not already support WPA. I would recommend using WPA. For WPA in the Macintosh, you can go to Airport help and search for WPA, after you have installed at airport software 3.3 or higher.
Some additional reading about WEP (an older doc, but just lets you know how long the problem has been around.
http://www.sans.org/rr/papers/68/149.pdf
Good document on caomparing WEP and WPA, plus securing wireless in general
http://www.giac.org/practical/GSEC/...y_Wong_GSEC.pdf
[end Edit]
Cost here ... let's see
Buy a WAP device that support WPA, such as Airport Extreme Base Station.
Upgrade the operating system on the Macintosh to Panther.
Possibly still have to buy an additional Antenna. If you do go this route with the AEBS, and don't need the modem port, I believe you can still add an antenna to the AEBS model that does not have the modem port or antenna port, by opening the bottom of the AEBS and conecting the antenna internally. That would save you $50, but I would confirm that before purchasing your AEBS.
Anyway good luck.