You should have no trouble with this in the default setup, but just in case, I would buy a hackable one so that you can reflash with a different distro for more advanced network configurations.
Keep in mind that I don't use windows, so my terms may be a little off. What you will want to do is set up your XP box for ipmasq, nat, "connection sharing" or something like that. Then get a DHCP server running. Plug the router into your XP box's ethernet port, and use the internet line on the back of the wifi router.
The wifi router will obtain a fake/local IP address from your XP box, and your XP box will nat/masq/share it to the internet.
Now you plug in a wifi card to your Z, and it obtains a different fake/local address, from a different 192.168 subnet, and the router will add a second layer of nat/masq connecting you to the internet.
2 layers of masq/nat have never caused me problems, not that I have done it much. It probably will work fine for you too.
In case it doesn't, what do you plan on doing? If it is 100% HTTP (web), you can skip the masq and just put a HTTP proxy on your XP box and then configure your Z to use that proxy. This would only be a last resort, but it would work.
Does this sound right to everybody else? I don't know much about XP, so maybe I am overestimating what it can do.