you get what you pay for when you get a little stick like that.
I have been researching usb sound cards and just recently bought an emu 0202 for 70 usb on craigslist. I upgraded from onboard sound on my dual core atom intel mobo. HUGE improvement. There actually is life below 60 Hz now.
I think for your sub you should see if your amp has an input and output so that you can set your cross over frequency on the sub to handle the lows that you speakers dont /cant.
Cheap soundcards will leave you desiring more due to their low cost DA converters and poor amplification. You should also pay attention to the output voltage of the sound cards and see if it compatible with the input of your HU or amp whichever you are using. My aux in in my car wanted about 2 Vrms so I didnt have much choice.
For a dac try to get a 24 bit dac if you can but 16 will do if its properly implemented.
Go to headfi.org to see what those guys use for usb sound cards. That will give you lots of good information.
I used this site to compare sound card outputs since they are given in odd units every time either Dbu Dbv or Vrms.
good companies to look into are m-audio, emu, and juli@ (forgot brand).
hope this helps.