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.
Product Specification: InterfaceUSB Transfer Speed3 Mbps Transfer Distance0 - 20m ControllerCSR8510 Driver SoftwareCSR Harmony Wireless Software Stack Dimension23 x 15 x 5mm Systems SupportedWindows XP / Vista / 7 / 8 / 10