WPA may not be supported by older WiFi hardware as it's a newer protocol.
Some cards/access points/routers can be upgraded via firmware reprogramming. Check Linksys' website.
The good thing about WPA is that it was fully baked as a standard before manufacturers adopted it. The same is not true about WEP, which resulted in minor difference in key generation between implementations. This is why, for example, the Apple Airport Express may not work with Linksys WiFi access points under WEP encryption. There are workarounds but no guarantees.
Other things you should do to secure your network include MAC filtering (allow only registered MAC addresses to access your net) and, perhaps, not broadcasting your SSID, so clients have to know the name of your net to access it.