Ok, in that case, the card you ordered might not fix the problem until you also get an 802.11g router. Sounds like the Airport Express is just acting like a client connecting to your router wirelessly, so it's also running at 802.11b speeds.

The good news is that a generic 802.11 g router is pretty cheep. Just stick to a normal one, don't go with some "speedboost" or other faster solution, since those all require special cards in the computers too. I don't have any particular recommendations, but if you want the Airport Express to also extend the range of the network a bit, find one that supports WDS.