I have recently purchased the SoundBlaster X-Fi HD soundcard and will like to share with you how to activate ASIO on Foobar and Win 7.
First you need to download ASIO4ALL plugin from http://www.asio4all.com/.
Microsoft wave synthesizer deactivation is required and the way to do this is to go to control panel->sound->playback devices and right click on the USB X-FI HD card and select properties. Then go to the advanced panel and uncheck "allow applicatios to take exclusive control of this device".
Close all applications that may be using the soundcard (including web browser) and starts Foobar. Go to Foobar menu under Perference->playback->ASIO virtual device and click on the button "add new devices" to select the soundcard you want to use for ASIO. you will need to select the option "always resample 44.1kHz<->48Khz".
Because the soundcard only supports 48khz and 96khz in sound format, you may want to use the resample plugin from Sox http://www.hydrogenaudio.org/forums/index.php?showtopic=67373
There is a ground pin at the back of this soundcard. Theoretically, if you pull a wire to connect this ground pin to your desktop metal casing, the electrical interference within the card should be reduced, and therefore translate to less noise and better sound quality.
Comparing the sound quality with my onboard HD Realtek ALC1200 using CAL! headphone, sound quality is improved; there is more bass and sound is more vivid(not so flat). However, the untrained ear will not be able to detect too much difference at all!
