A modern wireless router is usually compatible with 802.11b with certain limitations (see, for example, the Tech Specs and System Requirements at http://www.apple.com/shop/product/ME918LL/A/airport-extreme), at least with the appropriate settings. In order to get additional information about an AirPort Extreme base station setup, it may not be a bad idea to post a question under the AirPort discussions. See also the general information Recommended settings for Wi-Fi routers and access points - Apple Support about radio modes, etc.
The requirements for WPA with an AirPort card were:
- Mac OS X 10.3 or later
- AirPort software version 3.3 or later
If you wish to use WPA2, or if there is a problem with WPA or earlier security protocols, with radio modes or speeds, you may want to try an external wireless Ethernet bridge (connected to the Ethernet port of the computer) instead. There are such devices (also known as gaming or media adapters) from several manufacturers (Netgear WNCE2001 and Linksys WET610N are merely two examples). Do pay attention to the specifications before you buy. Typically, a wireless Ethernet bridge setup can be done via a standard web browser (it does not have to be on the computer where the device is to be used later). Special driver software is mostly not needed, thus allowing the wireless Ethernet bridge to work under almost any operating system (even on some very old Macintosh computers, as long as they are Ethernet-capable).