The previous answer may relate to this as well as I don't yet know enough about that service; but to put it simply - I like HDMI graphics cards because I (can)get full 5.1 and 7.1 sound for my TV and theater system. See below about the (can):
Getting that sound to the theater system can be a trick though if your limited on inputs to the device. Many times, becuase of DRM issues(related to blu-ray and cable recording), the HTS will only have one or two audio input devices and it is usually better to intercept the audio as early as possible.
If you use the PC speaker system to turn your PC into a theater system, then it probably wont be a problem, but if you preferr a separate HTS device with a DVD player they it can get tricky. Most only have one laser input and maybe a iPod or digital RCA input and the last two may not provide full HD sound quality.
You could put another high end audio card with component or laser output, but be forwarned certain devices can initiate DRM trips if the software, hardware, or cable device thinks your trying to "steal" premium content.
I always tell folks to go to "thegreenbutton.com" to look at problems other folks have, so they can avoid the pitfalls and get exactly what they want. Carefull planning can avoid a lot of problems.
Fortunately my TV has a good enough sound system, I can forget about my HTS when playing content on my PC through HDMI, but the cable OEM settup with not allow me to put sound through the slightly better HTS and get true 7.1 sound. They won't even let me put a audio card in that could ship the sound to the HTS. I could use the very capable 7.1 ability of the onboard motherboard audio, but I already have a fancy HTS so I'm not worried about it, I can add speakers later if I get to playing my PC blu-ray a lot.
So far their isn't much blu-ray content in the stores that I really want to watch so that'll be the day!