If you are not having problems using the ethernet, you should use your iMac that way instead of wireless.
To troubleshoot when wireless try all of the following suggestions:
Try the basics first:
Change your router channel number. Most times this works & is all you have to do.
Disconnect & reconnect your modem. unplug it for about 10 seconds. Plug it back in. Do the same for Apple’s routers. Wait for everything to reboot.
System Preferences>Network
Click the Assist Me button.
In the next window that pops up, click the Diagnostic button & do the necessary.
Research Knowledge Base for network problems that pertain to the OS that is currently installed on your computer. See these basic networking KB Articles: http://support.apple.com/kb/HT1401 AirPort troubleshooting guide
http://support.apple.com/kb/HT4628
Wi-Fi: How to troubleshoot Wi-Fi connectivity
http://support.apple.com/kb/HT2712 Using network locations in Mac OS X
Manually provided DNS server addresses are higher priority than DHCP's
http://support.apple.com/kb/HT1714 Solutions for connecting to the Internet, setting up a small network, and troubleshooting
============================
What to do when you can't connect to the Internet
Also, run the Airport Utility app which is located inside the Utilities folder.
=====================
If using a Linksys router, contact LinkSys Customer Support and/or post in their forums.
If using Apple's Airport, please re-post over in one of the AirPort Forums.
