Add wifi to Gateway 500S w/wired LAN

Hi all,
I have a basic question that in searching the wireless forum was not able to find anything exactly matching my topic but several posts appear to indicate it can be done. Recently have upgraded a Gateway 500S with memory and new HDD and have reinstalled Win XP Pro, applied Gateway and MS firmware/software/security updates.
We recently installed LinkSys WRT54G wifi router to our DSL broadband for gaming with Nintendo WII.

My question is twofold: Can a wifi card be added to the Gateway 500S and selectively used for wireless and then alternatively 'disabled' for wired-network access through the original network card. I suppose there is also the possibility of a dual function (wifi/wireless) card but basically I'm just wanting to enable wireless when it's available and a wired-connection is not. The Gateway 500S is a 5 yr old desktop Intel Pentium 4. It was able to support the upgrade from PCI IDE HDD to SATA I, 500Gb Seagate drive -- didn't require any additional drivers to be installed.

The second part of my post is, if the above is possible -- a bit of explanation of steps necessary to accomplish and any recommendations for single function wireless cards or if available, dual-function (wifi/wireless) network cards. I'm very new at wifi and if I've not even come close to how to accomplish this, I appreciate your time to let me know the best way to do this.
Thanks,
Greynold

Grumpy,
We recently setup wireless for younger son's WII gaming. I've been working on older son's desktop, he's at College and I thought, just while he's home why not setup his desktop for wireless access so he can use our DSL from his room. He'll be taking the desktop with him after Easter break.
Greynold

Thanks Grumpy,
I'll check it out this weekend. If I'm understanding correctly, I thought there might be simpler external wi-fi device to enable wireless.
Greynold

I thought also, just while he's home why not setup his desktop for wireless access so he can use our DSL from his room.