The front headphone jack on my computer works while the back jacks hiss, and the front and back microphone input both have a hissing sound that only goes away when I use the sound card that my Logitech G430 came with. 
 
I'm looking to upgrade to a Sennheiser HD 558 + Modmic 4.0 set-up in the near future for gaming, listening to music and Skyping so I'm looking for a decent external DAC/soundcard with a microphone input — if possible, from $50 - $100 — to clean up the hissing and perhaps improve the sound. Any suggestions? I was looking at the Audiotrak Prodigy Cube but couldn't find any recent reviews, does anyone have experience with it?
 
Also, my current onboard sound is an AC1150 from Realtek.

There is the Sound Blaster Omni, which is half the price of the E5 and comes with a built-in mic. A friend of mine uses it for his AKG Q701 and seems satisfied, and being on the other end of the call, I can tell you the mic is of good enough quality for voice chatting. Unless you are bent on getting the modmic, you could probably settle for the Omni's mic like my friend and save a few bucks. The Omni does have a mic input if you still want the modmic, though.

Thanks for the input, I think I'll go for the Omni, though I'll consider the E5 too,