Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Depends what you are trying to achieve? If you just want to use Bluetooth 4.0 then by all means buy a USB dongle.

If you want your 2009-2011 iMac to work like a modern iMac with all the cool functions of a inbuilt OEM WiFi Bluetooth system then a dongle is not for you.

Just buy one of the plug and play cards in post one for $$$ easy peasy.
 
I just want to connect my mouse (needs bt4) and my logi-keyboard
not Interessed on the rest...

The stock BT 2.0 unit can connect your KB & Mouse pretty well. No need BT 4.0

A USB BT dongle is cheap enough, and plug-and-play, too.

 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.