|Image by Adrian Whelan via Flickr|
For example, the Sennheiser PC 350 gaming headset has a requirement of 150ohms. I couldn't find any documentation online that specified the specifics on the headphone output of the Creative X-Fi Fatal1ty Pro (PCI edition) so I decided to contact support:
Thanks for your inquiry.
You should be able to use 150 Ohm headphones.
Worldwide Customer Response
And there you have it folks.