Hey guys,
I don't know if anyone's still around to listen, but I just wanted to thank everyone (especially user1234) for all the info - it's made my life much easier!
I have a main wireless router / wired switch to which a number of computers connect both wirelessly and wired. I also have another computer + xbox360 in a place where I cannot run a cable. Previously, my xbox 360 connected through my computer using ICS (on its wireless NIC), which was less than satisfactory - apart from anything else the NIC was crap and kept dropping the connection, when a laptop placed in the same place never did.
I bought the WRT54G on the basis of this thread and various related searches, then installed the Freeman hack of the Sveasoft software. Using these instructions my xbox 360 and computer were attached to a permenant, high-strength wireless network within 15 minutes! Thanks guys!
One thing that was different - I think I've seen this mentioned, but just in case: The Freeman version that I am using (1.04) allows you to select client-routed mode, which means the router connects as a client, and then I just use the inbuilt NAT to regulate the computers this side. The Linksys is placed in the DMZ of the main router, and the xbox 360 is placed in the DMZ of the Linksys, meaning I don't have to fiddle around with port forwarding etc. Also, I can still have laptops etc connect to the main AP - amazing!
In addition, I don't know if it's still an issue, but I was able to connect using WPA with no problems at all!
Thanks again for the help guys