Yeah, I think I might still need help. Everything in your post made sense to me, and all the settings I made on the WRT54G made sense to me, but for some reason it just wasn't happening. It even seemed to find the Airport and connect to it, but I just didn't have internet access on the computer connected to the Linksys.

*edit*
Well, I think I figured out why it's not working, but I can't figure out how to fix it. The issue is that I can't seem to match security settings on both devices, because Apple has a very weird assortment of wireless security settings on the Airport. For example, there's a choice for "WEP (Transitional Security Network)." What's the problem with this? Well, the Airport insists on a 13-digit key when using WEP. Every other device I've seen, however, refuses to let you enter more than 10. That makes no sense.

I've tried the other security modes but it's still clear that the Linksys is unable to connect to the Airport because I'm not able to match the security settings correctly.