wireless connection question

If the router you have is a wireless router, then yes you can connect to it wireless with the macbook whilst having the iMac connected using a wired connection at the same time.

If it isn't wireless, you'll need to either connect your Macbook using the wired connection, which can be done at the same time as the iMac is connected, or get a wireless router, such as an Airport Express, Extreme or Time Capsule. You could also get a non-apple wireless router if you so choose.

 

You can also turn you iMac into a very expensive router by going to system preferneces, then sharing, enable Internet sharing from Ethernet to airport. Then you have your own wireless router while still having your ac connected by a cable!