Need Help Getting WIFI to work w/ Linksys!!

I have a Linksys Wireless Router model # WRTP54G that I used to have hooked up to a PC laptop that worked great, but we've recently gotten rid of that laptop and reloaded up our old mac tower (G4 Quicksilver ... we have Tiger 10.4.11 on it). Now that I have the mac running, I can't get the wireless router to work. I hooked it all up and everything, and opened safari and typed in the IP address, but it just says that no internet connection can be made. If I just go straight from the modem to the mac with an ethernet cord, I have internet instantly--however, in doing this, nothing ELSE can get wireless (I.E. another laptop, Itouch, etc etc).

Help?

What type of encryption are you using for your network? Is the network showing up in your Airport menu?

Umm I'm sorry I'm not super great with mac (never used it before--I've always used PC) so I'm not sure how to answer those ?'s. Could you maybe explain a little, or is it too complicated? I'd probably understand because I'd at least consider myself an advanced user when it comes to PC, but like I said I'm just totally new to mac.

When I go into my network settings and I see "Built-in Ethernet" and "AirPort", the ethernet is active but the airport says "AirPort is currently turned off." So when I hit connect, it says the power is off but at the bottom where it says "Status: No AirPort Hardware Found". Now, I know I don't have a wireless card in this PC because the guy that reloaded it offered to put one in for us for like $25. But we didn't think we'd need a wireless card in it since it's just a desktop and it's always gonna be sitting here directly plugged into the router which is plugged directly into the modem. Will it not work that way? And by "encryption"....I'm assuming you mean do we have a password on our wireless??? No, we don't, it's totally open. (Which is a no-no...I know but oh well.)

So I guess now my question is....do I have to have a wireless card in this mac tower for me to be able to have wireless for anything else in the house--and if so, why won't it pick up the internet even though it's plugged into the router (which is plugged directly into the modem)?