Although wireless b should in theory be compatible with wireless g. In practise it isn't always the case. Particularly when using kit from different manufacturers.
By the sounds of it your are trying to set up a "adhoc" wireless network. On most routers you have the option of selecting what frequency to use. Do you have this option on the wireless g card? (May be under the set up disk interface) I don't think wireless b is compatible with g. Although on g you can select b.
Have you tried to access any other wireless networks?
If nothing else this will bump to the top