Guide to Mac-friendly wireless routers: WDS support
Linking wireless routers through WDS can provide a number of benefits, including extended signal range and an increased number of simultaneously connected devices on the same network. A number of third-party routers have issues when configured to use WDS with AirPort Base stations, however.
In general, third party wireless routers need to be set as the main router -- the unit that connects directly to an Ethernet network source then distributes to AirPort Base stations. For instructions on configuring the AirPort Base station as such, see Knowledge Base document #107454.
Among the routers that do support connectivity to AirPort base stations via WDS:
ResourcesLinksys WRT54G and WRT54G Though LinkSys scores low marks for Mac documentation and support, a few of the companies routers are compatible with the WDS scheme used by Apple. Frank Canzolino has posted some excellent instructions for configuring these routers with AirPort-compatible WDS.
Belkin F5D7230-4 and F5D723 Belkin scores high marks for Mac documentation and support as well as offering AirPort-compatible WDS connectivity through some of its routers. The company offers some basic setup instructions online.
Buffalo WGR-G54 Buffalo also offers excellent Mac documentation and support while also supporting AirPort-compatible WDS. Setup instructions can be found here.

I've been using An Airport Express with a Linksys WRT54GL for a few months now. The firmware of the Linksys is v4.30.5. I set its security mode as WPA-Personal.
My Airport Express says its wireless security mode is WPA2 Personal. Though if I click on the Security Options button, it says the encryption type is "WPA and WPA2."
I just thought I would chime in so people would not think that they have to use WEP.