Don't bother with Dongles for a reliable solution
You can either search this forum or use google to learn why they are less than ideal. Using a PC or Mac would be the more reliable solution. Using a dedicated internet streaming device would be good too (Roku, WDTV Live,etc.), but they have limitations to which content they are licensed to provide/stream to your TV(s). Some of the streaming boxes (mostly older ones though) do actually have composite (analog) connections, and could send at least some content to your older TV. Here's one example:
There are several methods to get internet content from your iMac/PC/etc. to your HDTV & regular TV. The regular TV is the hardest one to accommodate due to the distance involved, and the fact that it is not a digital device/lacks HDMI input. Most of the current crop of equipment will lack composite connections of the WDTV Live unfortunately. For shorter distances though, using HDMI output from your iMac to the main HDTV would be the ideal solution, and nearly bulletproof, provided your iMac has HDMI output. Wired always trumps wireless if both are available/practical. Another benefit of going w/ a wired HDMI connection is that you don't waste the local wifi bandwidth for other internet activities.
More expensive streaming solutions include using a Slingbox, which also lets you do a lot more outside the home with your paid content (CATV/Satellite) & broadband internet connections. See this here:
Let us know if you have any questions.